How to Make It
Preheat oven to 375°F with oven rack in the middle. Coat 16 wells of a silicone half-sphere mold or a miniature muffin pan with cooking spray, and set aside. (Place the silicone mold on a baking sheet.) The batter can wait if you need to bake in two batches.
Sift flour, almond meal, and cocoa powder into a medium bowl; set aside.
Place egg whites and sugar in the bowl of an electric stand mixer fitted with the whisk attachment. Beat on high speed (I use speed 8 on my KitchenAid) until mixture increases in volume and becomes white, very thick, and glossy, 2 to 3 minutes. Stop the machine to check by swiping a bit on a fingertip. The peak should gracefully flop over, and you should barely feel the sugar granules.
Return mixer to medium speed, and slowly pour butter down the side of the bowl, scraping down sides of bowl with a spatula as needed. Mix on medium-high until incorporated, 10 to 15 seconds. Reduce speed to low (2 on my KitchenAid).
Add half of the flour mixture to egg white mixture; beat on low speed until combined, gently scraping down sides of bowl with spatula. Add remaining half of flour mixture; beat on low speed until combined, about 30 seconds.
Spoon 1 tablespoon of batter into each well of the prepared mold to fill one-half to two-thirds full. Bake at 375°F until center of cakes spring back when touched, 12 to 13 minutes. Let cool in the mold for 10 minutes (the cakes will deflate slightly) before removing from mold and cooling completely on a wire rack. Store cakes in an airtight container up to 3 days.