In a large bowl, whisk together the flour, powdered sugar, baking powder, cinnamon, and a pinch of salt.
Add the cold butter and cut it into the flour mixture using a pastry blender or your fingertips until the mixture resembles coarse crumbs.
Grate the apple and add it to the mixture, then stir in the egg and vanilla sugar.
Knead the dough until it comes together, wrap it in plastic wrap, and refrigerate for 30 minutes to rest.
Preheat the oven to 350°F (180°C) and line a baking sheet with parchment paper.
Roll out the dough to a thickness of about 1/8 inch (3-4 mm) and cut out round shapes using a cookie cutter. Cut out a smaller circle in the center of every other cookie.
Place the cookies on the prepared baking sheet and bake for 10-12 minutes, or until lightly golden brown.
Let the linzer cookies cool completely. Spread chocolate spread on the flat side of the whole cookies, then top with the cookies with the cut-out centers. Sprinkle with grated dark chocolate before serving.
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