Preheat the ofen to 175 degrees. Melt the butter and stir it with the eggs and the sugar until light and fluffy. Stir in the milk. Fold in the flour, the cocoa and the baking powder. Stir in the rum.
Line three round cake tins with baking paper. Pour the dough into the cake tins and bake the three cakes in the preheated oven for 25 minutes. Stick a needle into the cake to see if the cake is ready.
For the buttercream: Mix the butter and the icing sugar. Cut the vanilla pod lengthwise, scrape out the core and mix into the cream together with the rum until the cream gets light and fluffy.
Spread the buttercream onto the three cooled cakes and set them onto each other with the cream in between. Spread buttercream on top of the cake and decorate it with flowers or rosemary twigs and cranberries.