Preheat the oven to 150°C (300°F) and line baking sheets with parchment paper.
In a large bowl, beat together the sugar and lard (or butter) until light and creamy.
Add the yogurt and mix until fully incorporated.
Gradually add the flour and vanilla sugar, mixing until a smooth dough forms.
Transfer the dough to a lightly floured surface and roll it out to approximately 5 mm (¼ inch) thickness.
Using a round cookie cutter or small glass, cut circles from the dough.
Place the cookies on the prepared baking sheets, leaving a small amount of space between them.
Bake for 12–15 minutes or until the cookies are set and lightly golden around the edges. Do not overbake.
Allow the cookies to cool completely on a wire rack.
Spread a small amount of plum or apricot jam onto the flat side of one cookie and top with another cookie to form a sandwich.
Combine the powdered sugar and vanilla sugar, then generously dust the assembled cookies.
Place the cookies in an airtight container and allow them to rest for several hours or overnight before serving for the best texture.