For the dough, sift the flour and baking powder together and pile up over the work surface. Make a well in the center of the flour and add in the sugar, vanilla sugar, lemon zest, salt and egg. Add the chopped butter and knead quickly into a smooth dough. Wrap the dough in plastic wrap and let rest for 20 minutes in the refrigerator.
Roll out the dough on a floured surface and cut out heart-shaped cookies from it. Press each cookie in the middle using a little sweetheart stamp and pour in some jam.
Line a baking sheet with parchment paper and place the cookies over it. Bake in a preheated oven at 190°C (fan: 170°C, gas mark 3) (approximately 375°F) for 10 minutes.