Prepare the Dough: In a mixing bowl, cream together the butter, powdered sugar, and vanilla sugar until light and fluffy. Add the egg, lemon juice, lemon zest, and salt, mixing well.
Incorporate Dry Ingredients: Sift in the flour and baking powder, then mix until a soft dough forms. Cover and chill the dough for 30 minutes.
Shape the Cookies: Roll the dough into small balls and coat them in granulated sugar first, then powdered sugar.
Bake: Preheat the oven to 180°C (350°F). Arrange cookies on a parchment-lined baking sheet, leaving space between each. Bake for 10-12 minutes, or until lightly golden on the edges.
Cool and Serve: Allow cookies to cool on a wire rack before enjoying their soft, buttery texture and bright lemon flavor.