To make these vanilla, replace the cocoa powder with flour and leave out the chocolate. Great additions are fruits on top like blueberries, raspberries, fresh currants, or blackberries. You can also put slices of peach, nectarine, or pear, just go with a firmer fruit so there's less water content and they cook through without any problems.
These can be baked in several different kinds of molds. The traditional is a flat rectangular. I like the taller rectangular cakes as seen in the pictures. Mini or regular muffin tins work as well. Just grease the muffin tin, there's no need for a cupcake paper. As these are rather rich, for regular muffins, I would only fill halfway. Adjust the baking time as needed.
Preheat the oven to 425F/225C.
In a bowl, whisk together the sugar, flour, cocoa powder, and almond flour. Then add the egg whites. Set aside.
Melt the butter over medium heat in a saucepan until browned bits form on the bottom. Take off the heat and stir in the chocolate pieces to melt. Add to the mixture above while still hot.
Fill your chosen mold (see note above) and press in a fruit if you'd like. After putting in the oven, immediately lower the temperature to 350F/165C. The time is going to depend on the type of mold used. The rectangular molds pictured took about 20 minutes. To tell when they are done, press lightly on top. It should spring back. You can also insert a toothpick into a non-fruit section. It should come out clean or with just little crumbs attached.