Start by making the shortbread dough: On a work surface, sift the flour into a mound. In the center, add the sugar, grated lemon and orange zest, and the cold butter pieces. Using your fingertips, work these center ingredients together until the mixture looks sandy.

Add the eggs one at a time to the center of the mixture. Once the eggs are fully incorporated, add the salt, vanilla extract or vanilla sugar, and baking powder (if using). Knead everything together until you have a soft, smooth dough that isn't sticky.

Shape the dough into a ball, wrap it in plastic wrap, and refrigerate for about 1 hour.

After an hour, take the dough out of the fridge and, using a rolling pin, roll it out on a floured surface to about 1/4 inch (0.5 cm) thick. (You can also roll the dough between two sheets of parchment paper.)

Use a round cookie cutter about 2 to 2 1/2 inches (5–6 cm) in diameter to cut out circles. Place a teaspoon of jam or chocolate spread in the center of each circle. Brush the edges with a little water, fold the dough over, and press the edges to seal, forming a half-moon shape.

Place the cookies on a baking sheet lined with parchment paper. Brush the tops with egg yolk and bake at 350°F (180°C) for about 15 minutes, or until they start to turn golden.

Let the cookies cool slightly before serving, and dust with powdered sugar.