Melt 20 g butter in a small pot and let it cool down. Beat the egg, 1 pinch of salt, 50 g sugar, 1 packet of vanilla sugar and 1 tbsp cold water with the whisks of the hand mixer for 5-7 minutes until creamy.
Mix flour, baking powder, 10 g starch and cocoa powder together and carefully fold into the dough, stir in melted butter drop by drop. Put the dough into a greased square springform pan (26 x 26 cm) sprinkled with flour and smooth it down.
Bake in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for 7-9 minutes. Remove the mould from the oven and let it cool down.
Mix 2 tbsp. milk and 15 g starch. Heat 140 ml milk in a small pot, do not boil. Dissolve 50 g chocolate coating in it. Add 1 pinch of salt, 30 g sugar and 1 packet of vanillin sugar to the milk and mix.
Mix the egg yolk and starch mixture, add to the milk and stir. Simmer gently over medium heat for 4-5 minutes while stirring. Pour the pudding into a flat bowl, place foil directly on the surface so that no skin forms.
Allow to cool to room temperature. Cream 100 g butter with the whisk of the hand mixer. Add the pudding to the butter by the spoonful and stir until smooth.
Carefully remove the cake from the tin and cut it in half horizontally. Carefully prick one half with a fork and sprinkle with coffee liqueur. Spread butter cream on top and place the second base on top, chill for about 15 minutes.
Chop remaining dark chocolate and milk chocolate and melt separately in 2 water baths. Cover the tartlets with the dark chocolate coating and decorate with the milk chocolate coating. Allow to set.
Cut into pieces and serve.