To begin, prepare the curd filling a day in advance. In a saucepan over medium heat, bring the whole milk to a gentle simmer without boiling. Reduce the heat and gradually stir in the buttermilk. Stir continuously until curds form, separating from the whey. Once curds have fully developed, remove from heat and strain through a fine mesh sieve or cheesecloth. Let the curds drain overnight in the refrigerator for the best consistency.

Preheat your oven to 200°C (392°F). Line a baking tray with parchment paper to prevent the tarts from sticking.

Take the drained curds and place them in a bowl. Lightly whisk the egg, then add it to the curds along with granulated sugar, almond flour, vanilla extract, and salt. Stir until smooth, ensuring all ingredients are fully incorporated. The mixture should be thick and creamy.

Roll out the puff pastry on a lightly floured surface to about 3mm (⅛ inch) thickness. Using a 10cm (4-inch) round cutter or a small bowl, cut out eight circles of pastry. These will form the base and top layers of the Mattentaart.

Place four of the pastry circles onto the lined baking tray. Spoon a generous amount of the curd filling onto the centre of each base, leaving about 1 cm (⅓ inch) border around the edges. Avoid overfilling to prevent leakage during baking.

Brush the edges of the pastry bases lightly with water to help seal the tarts. Place the remaining four pastry circles on top of the filled bases, gently pressing the edges together to enclose the filling. Use a fork to crimp the edges securely, ensuring a tight seal.

Whisk the egg yolk with a small splash of water. Brush the tops of the tarts evenly with the egg wash for a golden, crisp finish when baked. Using a small knife, make a tiny slit in the centre of each tart to allow steam to escape.

Transfer the baking tray to the preheated oven and bake for 20-25 minutes, or until the pastries are golden brown and puffed. Keep an eye on them to avoid over browning.

Remove the Mattentaarten from the oven and allow them to cool slightly on a wire rack for about 10 minutes. This allows the filling to set and enhances the flavour.

Serve warm or at room temperature. Mattentaarten are traditionally enjoyed with a cup of Belgian coffee or hot chocolate. For an extra touch, dust lightly with powdered sugar before serving.