1. Add the butter and sugar to a large bowl and mix until light and fluffy. Add the egg yolk and vanilla extract and mix until well combined. Mix flour, baking powder, and salt – add to the bowl and mix until streusels start forming, then knead with your hands until a smooth dough forms. Take about 2/3 of the dough out of the bowl and wrap into plastic wrap. Add the cocoa powder to the remaining dough and mix/knead it into the dough. If the dough gets dry because of the cocoa, add a splash of milk and continue kneading. Wrap into plastic wrap as well and place both dough pieces in the fridge for about 2 hours.

2. Preheat the oven to 150°C (300°F). Line a baking sheet with baking parchment and set aside. Roll out the dark chocolate dough on a lightly floured surface to a thickness of about 0.2 inches (0,5cm). Cut out little hearts* (0.8 inches/2cm), collect scraps, roll out again and cut out as many hearts as possible. Set them aside. Next, roll out the light dough to the same thickness as the dark chocolate dough. Use the same small heart cookie cutter and cut out hearts all over the rolled out dough – you want the hearts to be all over, evenly distributed in a somewhat chaotic pattern. Next, place the small dark chocolate dough hearts inside the hearts you just cut out to fill the heart-shaped holes. Use the rolling pin and press the two types of dough slightly together, then use a 5.2 inches (5,4 cm) heart cookie cutter* and cut out as many hearts as possible. Place the cookies with some space in between on the prepared baking sheet. If you still got small dark hearts leftover, use the cut-out light dough hearts and roll them out again. Repeat the whole process to get more heart cookies with the heart pattern. You want as many of these big heart cookies as possible, because you will not be able to make more with dough scraps that have both types of dough mixed.

3. When you placed the big heart cookies on the baking sheet, cut out a small heart in the center of each cookie. You can use the same small heart cookie cutter you already used, or a slightly larger one to get a bigger opening. Bake these cookies for about 16-18 minutes or until lightly browned around the edges. You don’t want them to brown a lot. Take them out of the oven and let them cool down for some time on the baking sheet, then transfer them to a wire rack and let cool down completely.

4. While the first batch of cookies is in the oven, collect dough scraps and roll them out again – you will get a somewhat marbled look now. Cut out large hearts (same size as the other ones) and place them on a baking sheet. Collect the scraps once more and cut out additional hearts – you want the same number of cookies you had when baking the first batch. Bake these cookies as well for about 16-18 minutes, then take them out of the oven and let them cool down.

5. To finish the sandwich cookies, add a small amount of chocolate spread on the marbled heart cookies and press cookies with a heart opening in the center on top to create heart-shaped sandwich cookies. Let them dry for some time, then store them in an airtight cookie box in a cool place – up to a week or so.