Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Grease square springform pan (24 x 24 cm) and dust with flour. For the chocolate dough melt 20 g butter in a small pot.
Leave to cool for about 5 minutes. Beat the egg, 1 pinch of salt, 50 g sugar, 1 vanilla sugar and 1 tbsp cold water with the whisks of the mixer for 5-7 minutes until creamy. Mix 30 g flour, baking powder, 10 g starch and cocoa.
Carefully lift under the egg mass. Stir in melted butter in a thin stream. Smooth the dough in the mould. Bake in a hot oven for 7-9 minutes. Remove from the oven and let it cool down in the mould.
For the cream, heat 140 ml milk in a small pot (do not boil!). Chop 50 g dark chocolate coating and dissolve in the milk. Add 1 pinch of salt, 30 g sugar and 1 vanilla sugar. Whisk 2 tbsp. milk, 15 g starch and egg yolk, then stir into the chocolate milk.
Simmer at medium heat while stirring for 4-5 minutes. Pour into a flat dish, place cling film directly on the surface to prevent skin formation. Leave to cool at room temperature. Stir 100 g soft butter with the whisks of the mixer for about 3 minutes until creamy.
Stir in the pudding by the spoonful.
Remove the cake from the tin and cut it in half vertically. Prick 1 base several times with a fork and sprinkle with liqueur. Spread buttercream on top, place 2nd base on top and chill for about 15 minutes.
Chop 300 g dark and milk chocolate coating separately. Melt in two hot water baths. Cover the cake with dark chocolate coating. Drizzle whole milk couverture in thin lines across the cake.
Use a knife or a wooden skewer to twist it lengthwise. Leave to dry.