Makes one 10" cake because my cake pan is 10". If your pan is a different size, scale up or down as needed.
Preheat oven to 325F, grease 10" pan, line bottom with a circle of parchment paper. Make sure the paper covers the entire bottom or it won't release and your cake will be a mess.
Mix the egg white powder with water, and let sit 5-10min to fully rehydrate before whipping.
Microwave chopped chocolate and butter in short bursts until fully melted. Avoid going too high: this needs to cool back to 95F-105F before you can use it. Mix in vanilla at very end.
Add the citric acid to the egg whites, beat in stand mixer to soft foamy peaks. Add sugar 1T at a time. Beat to medium-stiff glossy peaks. The mixture should not feel at all gritty when you pinch some between your fingers.
Once the whites are ready and the chocolate is down to 95F-105F, mix ~1/4 of the whites into the chocolate to lighten it. Then fold them together until homogoneous.
Pour into pan, bake 30-40min. It's done when the top is puffed, the color has become a bit dull, there are big cracks, and a toothpick inserted into the center comes out with sticky crumbs and not liquid batter.
Once it's out, run a knife around the edge to release it. Once it's fully cool you should be able to get the cake out by inverting it in one motion.
Refrigerate until 1-2hr before eating; it's moderately less tasty straight from the fridge because the butter is too cold to release flavor well.
Preheat oven to 325F. I use a baking stone.
Grease 9" springform pan and cover bottom with parchment paper. This is going in a water bath, and springforms don't do this well. There are a bunch of approaches here, but some good ones are to use an oven bag and then foil, silicone sleeves, or to use another pan just slightly larger than your springform. Aluminum foil should work if you had a wide enough roll, but mine isn't.
Beat eggs on high for ~5min; volume should ~2x.
Melt chocolate and butter in microwave. Mix smooth. Target 115F.
Boil full kettle.
Fold eggs into chocolate, 1/3 at a time.
Pour into springform pan. Put cake pan in roasting pan or baking sheet in oven. Pour kettle into outer pan.
Bake until center measures 140F, 25min.
Remove pan from bath, cool to room temp on rack. Refrigerate at
least a few hours, up to 4d.
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