I’ve entirely stolen this from cake queen Rose Levy Beranbaum. You’ll need:
6 ounces dark chocolate
yolks of 6 large eggs
whites of 6 large eggs
3.5 ounces (1/2 cup) sugar
3/4 teaspoon cream of tartar
Preheat your oven to 375 degrees Fahrenheit. Line a 1/2 sheet pan (17″ x 12″ x 1″) with parchment and apply cooking spray. Melt the chocolate most of the way in the microwave using several 10-second bursts of high heat, then stir until it’s smooth.
In the bowl of a stand mixer fitted with a whisk, whip the yolks and HALF the sugar together on medium-high until very thick and fluffy. Add the chocolate and stir on medium, scraping the sides as needed, until the mixture is homogenous. Transfer the mixture to another bowl and wash the mixer bowl and whip well.
Return the bowl to the mixer. Add the egg whites and whip them on medium until they’re foamy. Add the cream of tartar and whip to the soft peak stage. Slowly add the remaining sugar and whip to stiff peaks.
Stir 1/3 of the meringue into the chocolate mixture, then gently fold in the rest, 1/3 at a time. Scrape the mixture into the pan, spreading it evenly, and bake for about 15 minutes, until it springs back when tapped in the center. Cool on a wire rack for 5 minutes.
Dust the top of the cake with cocoa powder and loosen the edges of the cake by running a knife along the rim of the pan. Place another sheet of parchment over the sheet and flip everything over. Gently peel off the parchment paper and cool the cake completely. The cake can be refrigerated or frozen for later use.