Knead the flour, baking powder, egg, vanilla sugar, salt, sugar and butter first with the dough hooks of the hand mixer, then with your hands, to a smooth dough. Let dough rest for 30 minutes.
Roll out the dough on a floured work surface and cut out cookies. Place the cookies on a baking tray lined with baking paper.
Whisk the egg yolk with cream and brush the cookies with it. Sprinkle the cookies with decorating sugar and bake in the preheated oven (electric range: 175 °C/gas: level 2) for 12-15 minutes. Let the shortcrust cookies cool down on a cake rack.