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 form, then knead with your hands until a smooth dough forms. Wrap the dough in plastic wrap and place it 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 dough on a lightly floured surface to a thickness of about 0.2 inches (0,5cm). Cut out circles with a diameter of about 1.6 oz. (4 cm) and place them with some distance in between on the baking sheet. Collect scraps and roll out again to cut out more circles – you want an even number of cookies.

3. Place the baking sheet in the center of the oven and bake the cookies for 18-20 minutes or until very lightly browned. Take the cookies out of the oven and let them cool down on the baking sheet for a moment, then transfer to a wire rack and let cool down completely. If you could not fit all the cookies on the baking sheet, repeat the process.

4. Add a small amount of Dulce de leche to half of the cookies and press them together with a second cookie to create sandwich cookies. Dust with some confectioners’ sugar if you like (optional). Store the Alfajores in a cookie box in a cool place for up to 7 days.