If you want your decorative biscuit plain, use pastry flour instead of cocoa powder.
Heat up your butter to make a brown butter sauce and leave it to cool. Add the confectioner’s sugar, half of the egg whites, the flour and the COCOA POWDER. Add the other half of your egg whites.
Do not beat the mixture.
BAKING: Bake at 200-210°C (390-410°F).