1. Add the flour, confectioners’ sugar, cocoa powder, and salt to a large bowl and mix to combine. Add the butter, egg yolk, and vanilla extract and mix with the dough hooks of a hand mixer (or with the food processor). As soon as the mixture starts looking like streusels, set about 1/2 a cup aside, and then continue kneading the rest until you get a smooth dough. Wrap the dough in plastic wrap and place it together with the streusels in the fridge for about 30-45 minutes.

2. Preheat the oven to 180°C (350°F). Line a baking sheet with baking parchment and set aside.

3. Roll out half of the dough thinly on a lightly floured surface – leave the rest in the fridge. Use a serrated pastry wheel* and cut strips with a width of about 5 cm, then cut these strips into triangles (see pictures). Place the triangles on the prepared baking sheet with some space in between. Brush half of the triangles with milk and sprinkle them with some streusels.

4. Bake the cookies in the preheated oven for about 10-11 minutes. Take the cookies out of the oven and let them cool down briefly on the baking sheet, then transfer them to a wire rack and let them cool down completely. Repeat the process with the remaining dough and streusels, collect leftover dough and roll out again. If you are out of streusels, you can turn leftover dough into streusels by crumbling it.

5. Warm the quince jam slightly to make it easier to work with. Spread a little jam on each cookie (without streusels) and place a matching cookie with streusels on top and press them together. Dust the finished sandwich cookies with a little confectioners’ sugar. Store the biscuits in a cookie box that closes well and leave them in a cool place. Eat them within a week.