Place the baking mix, egg, fat and 125 ml water in a bowl and work into a smooth dough with the whisk of the hand mixer. Spread the dough on a baking tray lined with baking paper.
Bake in the preheated oven (electric oven: 200 °C/ gas: level 3) for about 20 minutes. Let them cool down. Cut the cake into 3x6 cm rectangles. Dissolve the chocolate coating in a hot water bath. Dip the rectangles with the smooth surface halfway into the chocolate coating, place them on a cake rack and let them set.
Dissolve the white couverture in a water bath, fill into a freezer bag, cut off a small corner and decorate the rectangles with a domino pattern.