Preheat the oven to 180°C and line two 12-hole cake tins with paper cases.
Whisk eggs and sugar until light and fluffy.
Carefully fold in the flour and melted butter.
Pour the mixture into the prepared paper cases.
Bake for 15-20 minutes until golden and a skewer comes out clean.
Cool cakes on a wire rack for 10 minutes before removing from the tin.
Beat the icing butter until soft, then mix in half the icing sugar.
Add remaining icing sugar, cocoa powder, and milk, then beat until creamy.
Spread the buttercream on the cooled cakes and decorate with chocolate buttons.