For the lemon hearts, beat the egg yolks, sugar and vanilla sugar on high speed for 1 minute until creamy. Add the baking powder and lemon zest. Add enough of the almonds to make a firm paste or so that the dough hardly sticks.
Roll out the dough on a work surface covered with almonds or powdered sugar to a thickness of about half a centimeter. Cut out hearts and place on a baking tray lined with baking paper. Bake at 175-200° C for about 10 minutes.
For the icing, sift the powdered sugar and stir smooth with the juice of one lemon to make a thick mixture. Immediately after baking, brush the lemon hearts with it.