Quickly knead butter in pieces, sugar, flour, egg yolk and 2 tablespoons of ice-cold water with the dough hooks of the hand mixer to a smooth dough. Form dough into a ball, wrap in foil and let rest for about 20 minutes.
In the meantime, heat the double cream in a small pot at lukewarm temperature. Pull the pot off the stove. Chop the chocolate and melt in it while stirring. Roll out the dough on a work surface sprinkled with flour to a thickness of about 2 mm. Cut out 12 circles with wavy edges (each 8.5 cm Ø), place them in 12 greased hollows of a muffin baking tray and press them on. Knead the rest of the dough together, roll out again on the floured work surface approx. 3 mm thin and cut out 12 circles with a wavy edge (6.5 cm Ø each).
Fill the chocolate cream into the dough bowls, place the dough circles on top and press the edges lightly with your fingers. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) for approx. 20 minutes until golden brown. Remove and let cool on a cake rack. Turn out the tartelettes from the tin and dust with icing sugar.