Mix the soft butter with sugar until very creamy, stir in the eggs one by one for about 30 seconds, stir in the flour mixed with baking powder, cornstarch, cocoa and rum.
Spread the dough in a prepared square springform pan and bake for about 30 minutes (stick test).
Let cool in the mold, then spread the topping on, shape waves or make dents with a spoon, 2-3 tablespoons each. Drizzle with egg and chocolate liqueur.
Covering:
Beat the whipped cream with cream stiffener according to the instructions, stir in the quark, icing sugar and egg liqueur.