For the dough, mix 350 g flour and baking powder. Add sugar, 1⁄2 TL salt, egg and butter in pieces. First knead with the dough hooks of the mixer, then quickly with your hands to a smooth dough.
Form into a flat brick, wrap in foil, chill for about 1 hour.
Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Line two baking trays with baking paper. Halve the dough. Roll out halves one after the other on a lightly floured work surface to a thickness of 3-4 mm.
Use a round cookie cutter (4.5 cm Ø) to cut out cookies, knead the dough again and again, cool if necessary, roll out and cut out.
Put cookies on the trays. Bake in the hot oven for 8-10 minutes one after the other. Take them out and let them cool down on a cake rack.
For the filling, whisk butter, icing sugar and vanilla pulp with the whisk of the mixer for about 5 minutes until creamy, stirring in plum water (alternatively without alcohol: apple juice or water).
Pour into a piping bag with perforated spout. On 2⁄3 of the cookies squirt cream rings. Pour some plum jam into each of the rings with a teaspoon. Put 2 buttercream cookies on top of each other. Place the rest of the cookies on top and press them down carefully.
Allow to set in a cool place for 2-3 hours.
Chop the chocolate coatings separately. Melt each with half the coconut oil in a hot water bath. Dip half of the biscuits into the dark chocolate coating and lift them out again with a fork. Drain a little, place on baking paper and let dry.
Dip the remaining biscuits into the white chocolate coating and let them dry.