Place the flour with the sugar on the work surface, slide in the egg and yolks. Sprinkle the lemon zest on top. Knead the butter in small flakes, then knead the ingredients into a smooth dough. Wrap in cling film and let rest in the refrigerator for 1 hour.
Then knead again briefly, roll out and cut into 30 - 40 squares (6 x 6 or 5 x 5 cm). Mix the marzipan mixture with the ground walnuts, egg yolk and rum and place a dab in the middle of each of the dough squares. Fold the corners towards the middle, place on a baking sheet lined with baking paper and brush with whisked egg yolk. Bake in the oven preheated to 180 - 200 ° C for about 15 minutes.
Mix 3 tablespoons of powdered sugar with a little lemon juice to a spreadable glaze and cover the love letters with it. Knead the rest of the marzipan mixture with the powdered sugar, roll out and cut out as many small hearts as there are biscuits. Dust with cocoa powder and place on the love letters. Press lightly and allow the casting to dry completely.