This general approach works well for many kinds of fruit, though strawberries have quite a bit more water in them than fruits like plums or even raspberries, so a little more reduction is called for here. The recipe comes by way of Rose Levy Beranbaum, the queen of cakes. She calls this preparation “strawberry conserve”, and uses it to flavor buttercreams and spread on cake rolls. Compared to a typical jam or store-bought filling it has a good deal less sugar and more real fruit flavor, and there’s nothing wrong with that. You’ll need:
4 pounds fresh or frozen (no sugar added) strawberries
8 ounces (generous cup) sugar
12 ounces (1 1/2 cups) water
I’ll get to that as we go along. First wash and hull the strawberries. Now there’s a cheerful image of spring!
Next, combine the water and sugar in a large pot.
Bring that to a boil and add as many strawberries as will cover the bottom of the pot, no more.
Boil them for 60 seconds. You’ll notice that they will soften considerably and will lose some of their color. Remove them to a colander set over a larger bowl (to capture the juice/syrup that runs off).
At this point, reduce the syrup that’s left in the pan by simmering it. When the syrup reaches 208 degrees Fahrenheit, you’re at the right point to add more fruit (we all remember that syrup temperature is also a measure of sugar concentration, yes?). Repeat the process:
Every so often, return the syrup from the bowl that’s under the colander to the pot for reduction. This is what I really like about the way RLB treats fruit fillings generally. She never wastes a drop of flavor.
Continue that…blanching and reducing the syrup until all the berries have been given their first cooking. When the syrup has been cooked to 208 F for the final time, return the berries to the pot and simmer another 10 minutes or so.
RLB recommends continuing to cook until the conserve reaches a final volume of 3 1/2 cups, mainly because she preserves hers by canning it in small quantities. Me, I don’t do that. I generally stop the cooking before the conserve is reduced quite that much, because every minute the fruit cooks the softer it gets and the more color it loses. And I find there are a lot more possibilities here if I don’t cook it all the way down.
For instance, I can strain the fruit out and use it as it is, perhaps as a kind of cooked “whole fruit” filling between layers of cake. Or, lightly chopped, as a garnish to topping. The syrup I can repurpose for any number of things: as a flavoring for an icing or frosting, perhaps in a cocktail, as a drizzle in a plated dessert, or just over ice cream. More reduction may be needed depending on what I want to do.
Alternately I can lightly mash or chop the semi-soft fruit, apply a thickener and use it as a spread, as I shall do in the case of my modern stack cake. Here I’ll generally use gelatin, which I prefer over cornstarch or arrowroot, but either of those are fine choices. I use one .25-ounce packet (combined with about a tablespoon of ice water, then melted into the hot conserve) which results in a spreadable filling. I’d say it would take about 1/4 cup of a starch thickener to do the same job. The primary object is to bind up the syrup such that it doesn’t soak into a cake layer too readily (though it will do that to some extent…head that off if you wish by applying a scraping of buttercream to the cake layers both above and below your filling).
Now I’d say it’s time to make a stack cake!