DOUGH:Beat the egg yolk, butter, sugar, vanilla sugar, and milk until light and fluffy. While mixing, gradually add the flour mixed with baking powder, salt, lemon juice, and lemon zest. Wrap the dough in plastic wrap and refrigerate for 2 hours. FILLING:Whip the egg white with sugar and vanilla sugar until stiff peaks form. Gently fold in the ground almonds (or walnuts / hazelnuts), grated lemon zest, and rum. SHAPING THE COOKIES:Roll out the dough to a thickness of about 2–3 mm. Use a round cookie cutter to cut out circles approximately 6 cm in diameter. Place a teaspoon of filling in the center of each dough circle, then pinch the top and bottom edges together or twist them to resemble a candy shape.
Bake the cookies at 160°C (320°F) for 15–20 minutes.
Dip the cooled cookies in milk, then roll them in granulated sugar.
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