Method:
To prepare the cocoa sponge cake, first pour the egg whites into the bowl of a stand mixer and start whisking at medium speed. Add sugar a little at a time, increasing the speed of the mixer. The egg whites must be whipped until they are shiny
While the mixer is running, sift the flour together with the starch and cocoa powder. Repeat the operation twice then set aside. Meanwhile, melt the butter in a saucepan and let cool.
When the egg whites are whipped, pour the yolks into the mixer while in operation. After having added the yolks, turn off the stand mixer: the mixture obtained should flow slightly from the whisk. Pour part of the mixture into a bowl and add part of the powder.
Pour in the melted butter and mix; then add this mixture to the rest of the whipped part, add the remaining powder and stir gently from the bottom to the top. Now place the pastry rings with a diameter of 7 inches (18 cm) and a height of 1/3 inch (8 mm) on a drip pan lined with baking paper: you can bake 2 at a time and pour the dough inside.
Level the surface with the back of a spoon and bake in a static oven preheated to 430° F (220° C) for 6-7 minutes. Once cooked, remove from the oven and let it cool, then use a small knife to remove the sponge cake from the ring. Place the cocoa sponge cake on a rack to cool completely before using it for your creations.