This is a classic génoise save for the fact that 25% of its flour volume has been replaced by cocoa powder. Otherwise it’s pretty much the same. Start by preheating your oven to 375 degrees Fahrenheit and sifting together your dry ingredients: the cake flour, cocoa and salt.
That done, turn to the wet stuff. Combine the sugar (here a “wet” ingredient), egg and vanilla in the bowl of your stand mixer.
Whisk them all together a bit…
…then place the bowl over a large saucepan that’s got about an inch of boiling water in it. But, you know, that’s actually on the stove.
And whisk. The idea here is to warm the egg-and-sugar mixture just to the point that it feels like a warm bath. This will help the eggs to whip up high and with small, even bubbles.
You don’t want this hot. If you don’t have a thermometer, dip in your finger and test it. It should feel like a nice warm bath. Ahhh. At least 110 but no more than 120 degrees Fahrenheit, in other words.
Now whip. On medium-high for about eight minutes. At that point start checking your foam. You want it so a medium ribbon falls and the blobs the ribbon leaves most remain on top.
Plop…and it should pretty much stay there.
Alright then. Pour off about 1 cup of the foam into a small bowl or ramekin that has your cooled, melted butter in it. You can’t see it but it’s there, trust me.
Stir that all together with no particular delicacy, scooping up from under to make sure the butter is all incorporated. This will lighten the butter and help it incorporate into the batter.
Pour it in.
Now gently sprinkle on your flour mixture…
…and either fold or gently whisk it in. Here I’m doing the whisking method, you want to gently rotate the whisk around the outside of the bowl. Don’t beat it with any strength or you’ll overly deflate the batter. As it is you’ll lose a little of your volume, but that’s expected.
Scrape it into your prepared pan or onto a parchment-lined baking sheet if you want to bake it in that form.
Bake it 30-40 minutes until it’s firm in the center. Remove it to a rack to cool for 5-10 minutes, then gently remove the springform sides and peel off the parchment.
As it cools it may fall a little in the center in the first few minutes out of the oven. This is normal for spongecakes, don’t worry. If it goes a little concave you can always trim the outer edges off to even it out. The thing you don’t want of course is a total collapse. Anyway, place a piece of parchment in the top.
Flip the cake over and remove the pan bottom and parchment.
Then flip it back. Allow it to cool completely. This will store a day at room temperature or freeze for up to two months.