Mix the butter with the powdered sugar and egg yolks until foamy, stir in the flour and baking powder. Beat egg whites with sugar until stiff and fold into the prepared dough. Remove some of the batter and fold in the cocoa.
Butter a large round sponge cake tin or a springform tin with a chimney and sprinkle with breadcrumbs. Put half of the light dough into the mold, then put the dark dough on top and finally the remaining light dough, pull through briefly with a fork.