Rose Levy Beranbaum has done a great deal to popularize the so-called "one bowl" mixing method. She employs it in virtually all her cake recipes, and these chocolate layers are no different. Start by combining the boiling water and cocoa powder:

Whisk until smooth and set aside to cool completely.

Once that's done, prepare your pans and set the oven to 350. Next, sift your flour into the bowl of a mixer fitted with a paddle.

Add the rest of the dry ingredients and stir on low to combine.

The last step before mixing is to prepare your egg mixture. Combine 1/4 of the cooled cocoa mixture with your room-temperature eggs...

...and wreck'em.

Now it's time to mix. Add the butter and the rest of the cocoa mixture to the dry ingredients.

Stir on low for perhaps 30 seconds to moisten everything. Then turn the mixer up to medium and beat for 1 1/2 minutes until the batter is creamy and light in color. Scrape the bowl, then start adding the egg mixer in three additions.

Beating the batter on medium for 20 seconds after each addition of egg, scraping the bowl well afterward. When all the egg mixture is incorporated, scrap the batter into your layer pans. You'll be putting about 1 lb. 5 ounces of batter in each. Spread it even with a spatula.

Bake for 25-35 minutes until the layers are springy to the touch. Cool the pans on a rack for 10 minutes...

...then turn them out onto a greased rack for ten minutes. I'll give you a word of warning: these layers can be a bit sticky on their surfaces. As you can see, I lost some of the skin of the layers when I peeled off the parchment. This is not a big deal, just don't leave the turned-out layers on the rack much more than 10 minutes, or you may have a more serious sticking problem on your hands.

Once cool, wrap the layers in plastic wrap and store in the refrigerator.

UPDATE: Reader Chana says:

One bowl? I count three for that chocolate cake (cocoa, flour, eggs), and then there's the measuring cup for the hot water, and the sifter. It's par for the course (of course), but a one-bowl cake it ain't. Just saying.

Very true, Chana, the terminology isn't especially apt in this particular case, but that's just what it's called. Broadly, the "one bowl" method applies to a mixing method that incorporates the butter directly into the dry ingredients before the wet ingredients are added. So OK, this recipe adds more than just the butter to the dry ingredients. You've got me there too. But you know, this mixing strategy is also known as "quick method", though I'll grant you it isn't especially quick in this case, either. But then it's also called the "blending method." Happy now? Sheesh!

I wasn't expecting to be back in Rose Levy Beranbaum territory so soon, but when you're talking about the great American layer cake, it's clear who the go-to lady is. Here's her classic chocolate butter cake, adapted from the Cake Bible:

2.25 ounces unsweetened cocoa (Dutch process)
8.25 ounces boiling water
3 large eggs (at room temperature)
2.25 teaspoons vanilla extract
8.25 ounces sifted cake flour
10.5 ounces granulated sugar
1 tablespoon baking powder
3/4 teaspoon salt
8 ounces unsalted butter, softened

First, in a small bowl, combine the boiling water and cocoa and whisk to combine. Allow the mixture to cool to room temperature. Meanwhile, preheat your oven to 350 and prepare two 9" layer pans according to the How to Prepare a Cake Pan for Baking post under the Techniques menu. Combine the dry ingredients in the bowl of a mixer fitted with a beater attachment and stir on low 30 seconds to combine. Crack the eggs into a small bowl.

When the cocoa mixture is cool, mix your batter. Add 1/4 of the cocoa mixture and the vanilla to the eggs and whisk lightly to combine. Set aside. Add the butter and remaining cocoa mixture to the dry ingredients and mix on low 30 seconds or so until all the ingredients are moistened. Scrape the bowl. Turn up the mixer to medium and beat 1 1/2 minutes until lighter in color and smooth. Add the egg mixture 1/3 at a time, beating 20 seconds between each addition and scraping the bowl down. Divide the batter into prepared pans and bake 25 to 35 minutes until the centers of the layers spring back lightly when touched. Let cool ten minutes then turn the layers out onto greased racks for 10 more minutes. Flip the layers right-side up to cool the rest of the way. Wrap airtight or freeze if you wish.

There is a plethora of photos in this post — 18 to be precise, and that's a lot for a post on something as mundane as a cake layer. However as I've written before, it's the simple things that demand the most attention. Get your layers wrong and your cake will join the ever-growing heap of mediocre bakery churned out by the world each day. Get them right and the result will be a kind of edible poetry...for the taste of buttery, made-from-scratch layer cake touches a spot deep down inside those of us who grew up with them, bringing back feelings of childish comfort and joy we're only rarely able to get back in touch with. Good cake, therefore, is a momentary ticket right back to some of the happiest moments of our youth. Is that not something worth striving for? Let's do it!

Start with your liquid ingredients. Put your (warm) egg yolks in a bowl, adding most of your (warm) milk and vanilla (need not be warm).

Wreck'em.

Put the sugar in the bowl of your mixer.

Then add your sifted flour. Me, I put a strainer on top of my mixer bowl, which I place on a scale. This makes it easy to weigh out the flour, then sift it directly into the bowl.

Once the flour is weighed out, spoon your leavening right on top (it's always a good idea to sift your leavening with your flour, to make sure it gets evenly distributed). Stir it in a bit with your fingers, then push the mixture right on through the sieve (taking it off the scale first, of course).

Sprinkle in the salt afterward because the larger crystals will otherwise get hung up in your sieve/sifter.

Put on the beater attachment and turn the mixer on low, give the whole thing a good stir.

Add your very soft butter in large pieces (a spatula is better than a hand...this butter only held its shape for an instant [this photo] before it oozed all over the place).

Add the reserved portion of your (nice warm) milk...

...and beat on medium speed until it's well combined and you've got the basis of your emulsion started, like so:

Add a third of your egg mixture...

...beat it in...

...and scrape the bowl thoroughly (especially the very bottom by the dimple).

Keep going like that: add, beat, scrape...add, beat, scrape...until all your ingredients are in. Yes, this will take you a little while, but great cake can't be rushed. Be patient. In the end you want a nice, smooth and glossy batter that looks about like this:

Using your scale, divide it evenly into prepared pans (instructions on how to prepare your pans are also over there on the right). Even it out a bit with a spatula, but don't worry about getting it perfectly smooth.

Bake according to directions until the center doesn't "slosh" anymore and the top is golden. Let the layer cool in the pan on a wire rack for ten minutes or so...

...then turn the layer out onto the rack to cool for at least an hour.

And your layers, they are done.

This recipe is based on Rose Levy Berenbaum's excellent All Occasion Downy Yellow Butter Cake, adapted slightly to my own tastes and techniques.

Yellow Butter Cake

6 large egg yolks
1 cup milk
2 teaspoons vanilla
13 1/2 ounces (3 cups) cake flour, sifted
10 1/2 ounces (1 1/2 cups) sugar
1 tablespoon plus 1 teaspoon baking powder
3/4 teaspoon salt
12 tablespoons unsalted butter, softened

Begin by preheating your oven to 350˚F. Combine the yolks, about 1/4 cup of the milk and the vanilla, beating them lightly to blend. Pour the sugar into the bowl of a stand mixer, then sift the flour and the leavening together into the bowl. Add the salt. Turn the mixer on low and blend the dry ingredients for about one minute. Add the soft butter, continuing to stir until it's mostly blended in, then add the rest of the milk. Slowly raise the mixer speed to medium and beat for 1 1/2 minutes until the batter is smooth and uniform. Scrape down the bowl.

Now begin to add the yolk mixture. Pour in about 1/3 of it, beat the mixture at medium speed for 20 seconds, then thoroughly scrape the bowl, paying particular attention to the bottom by the dimple. Continue on in that fashion until the entire yolk mixture has been incorporated. Divide the mixture between the two pans. Bake for 25 minutes, then check the layers. If the middle seems slightly sunken and mushy to the touch, continue to bake for another 7-10 minutes.

When fully baked, place the layers, in the pan, on a wire rack to cool. After 15 minutes, turn them out onto the rack to cool completely, about another hour.