Cut the couverture into pieces and put them in a bowl with the butter, melt in a hot water bath, stirring from time to time. Leave to cool a little. Separate the eggs. Beat the egg yolks with icing sugar and vanillin sugar until creamy.
Stir in the chocolate mixture and whipped cream. Beat the egg whites until stiff, pour onto the chocolate mixture, sift the flour over it and fold in.
Line a flat, rectangular ovenproof dish (approx. 15x15 cm) with baking paper. Pour in the dough. Bake in a preheated oven at 180 degrees (gas. level 2-3, fan oven: 160 degrees) for 40-45 minutes.
Let it cool down.
Bring blueberries to the boil in a pot with the liquid and cinnamon. Mix the starch with a little cold water. Stir into the blueberries and bring to the boil again while stirring. Let the sauce cool down, stirring from time to time.
Turn out the chocolate cake from the mould. Cut into cubes and dust with cocoa. Arrange on plates together with the blueberry sauce and sprinkle with pink pepper.