Sieve the flour onto the work surface. Place 1 egg yolk in the flour bowl. Put sugar, vanillin sugar, salt and fat flakes on the edge of the flour. Chop well with a wide long knife.
Work quickly with cool hands to a smooth dough. Wrap in foil and chill for about 1 hour. In the meantime, chop the plums very finely and mix with the apricot jam. Roll out the dough on a floured work surface to a thickness of about 1/2 cm.
Cut out hearts (4 cm Ø). Spread half of them thinly with the jam. Cut out the middle of the remaining biscuits with a small heart shape (2 cm Ø). Place on the coated biscuits.
Press the edges lightly. Whisk the remaining egg yolks with a tablespoon of water. Spread the biscuits with it. Bake in the preheated oven (electric: 175 °C/ gas: level 3) for 15-20 minutes. Results in about 30 cookies.