Beat 250 g butter and 250 g sugar until fluffy. Stir in eggs one after the other. Mix flour and baking powder and stir in briefly. Spread half of the dough evenly with a dough card on a greased baking tray (38 by 34 centimetres).
Stir cocoa into the remaining dough and carefully spread it on the light-coloured dough. Drain the apricots well and place them on the dough, except for eight to ten halves for decoration. Bake in the preheated oven (electric oven: 175°C/ gas: level 2) for about 35 minutes.
Let it cool down. In the meantime, stir the pudding powder, remaining sugar and six tablespoons of milk until smooth. Bring the remaining milk to the boil, stir in the pudding powder and bring to the boil again briefly. Place in a bowl, cover with cling film and allow to cool.
Whip remaining butter until creamy white. Stir the pudding briefly and add to the butter by the spoonful, stirring continuously. Spread the butter cream on the cake and refrigerate until the cream is firm. Melt the chocolate coating and fat in a hot water bath.
While stirring, allow to cool slightly and spread on the cake. Use a cake comb to draw wavy strips through the chocolate. Let the chocolate set. Cut the remaining apricots into slices.
Cut the cake into 24 pieces and dust with icing sugar. Serve decorated with apricot slices and lemon balm.