Mix flour and baking powder. Add butter in flakes, egg, sugar and vanilla sugar and knead briefly. Halve the dough. Knead cocoa and milk into one half. Wrap both doughs in foil and chill for about 30 minutes.
Roll out and shape each dough into a rectangle (approx. 25 cm long and 1.5 cm thick) on a floured work surface. Cut the light and dark dough into 4 strips each and chill for about 15 minutes. Separate the egg. Brush the dough strips with egg white. Put 2 light and 2 dark strips together like a chessboard. Chill for 1-2 hours. Cut into approx. 30 slices and place on 2 baking trays lined with baking paper. Bake one after the other in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for about 12 minutes. Whisk egg yolk and cream.
Put 2 light and 2 dark strips together like a chessboard. Chill for 1-2 hours. Cut into approx. 30 slices and place on 2 baking trays lined with baking paper. Bake one after the other in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for about 12 minutes. Whisk egg yolk and cream. Take out the pastry and immediately brush with the whisked egg yolk, allow to cool