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 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 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. Cut out round cookies with a cookie cutter and place them with some space in between on the prepared baking sheet. Brush half of the cookies with milk and sprinkle them with streusels.

4. Bake the cookies in the preheated oven for about 10 minutes. Take the cookies out of the oven and let them cool down briefly on the baking sheet before transferring them to a wire rack. Let 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. Cut the chocolate nougat into small pieces, add it together with the Biscoff spread to a small bowl and melt it in the microwave. Mix to combine. Spread small portions on the cookies without streusels and then press a cookie with streusels on top to create sandwich cookies. Dust the cookies with confectioners’ sugar to your liking (optional). Store the sandwich cookies in a cookie box in a cool place. Eat within a week.