Grease a square baking tin (approx. 24 x 24 cm) and dust with flour. Preheat oven (electric cooker: 180 °C/circulating air: 160 °C/gas: see manufacturer). Coarsely divide 300 g chocolate into pieces. Cut butter into pieces.
Melt both in a small pot at low heat while stirring. Take them off the stove.
Chop 100 g chocolate. Mix flour, cocoa and baking powder. Beat the eggs, sugar and 1 pinch of salt with the whisk of the mixer for about 5 minutes until thick and creamy. Stir chocolate butter into the egg mixture.
Alternately stir in buttermilk and flour mixture. Finally fold in chopped chocolate.
Fill half of the dough into the baking tin, smooth it down. Spread the waffles side by side. Add the rest of the dough and also smooth it down. Bake in a hot oven for approx. 25 minutes (stick test, see page 121).
Let the cake cool down for at least 4 hours.
For the topping, stir cream cheese and milk in a bowl with a tablespoon until smooth. Sift icing sugar over it. Stir with the whisks of the mixer for about 3 minutes until creamy. Cut the brownie into pieces and spill the topping on them.
Sprinkle with colourful sugar pearls, sugar hearts or sugar sprinkles as desired.