Preheat oven (electric cooker: 180 °C/circulating air: 160 °C/gas: see manufacturer). Line the bottom of a square springform pan (24 x 24 cm; alternatively round, 26 cm Ø) with baking paper. Cream butter, salt, brown sugar, sugar and vanilla sugar with the whisks of the mixer. Stir in 1 egg. Mix flour with baking powder. Quickly stir into the butter-egg mixture with chocolate drops. Press the dough in the form to a base. Pre-bake in a hot oven for approx. 18 minutes. In the meantime prepare the brownie dough.
Melt butter with dark chocolate (in pieces) at low heat. Beat the eggs, brown sugar and salt until light creamy. Stir in chocolate butter. Mix flour, baking cocoa and baking powder, stir into the chocolate mixture. Spread on the cookie base and bake for about 15 minutes. Let it cool down for about 6 hours.
Quickly mix butter, brown sugar, sugar, vanilla sugar, milk, salt and flour to a smooth dough. Stir in chocolate drops or another ingredient for baking. Spread on the cooled brownie and chill for approx. 30 minutes.