Beat the butter with the sugar and while stirring continuously add the eggs, flour with baking powder and lemon zest. Pour the mixture into a rectangular tray, greased and floured in advance and bake until ready in a preheated moderate oven.
Cut the ready layer into rectangular chunks while still warm. Prepare the chocolate syrup by melting the butter and sugar with the cocoa, water and vanilla. Let the syrup come to a boil, remove from the heat and add the rum.