In a large bowl, whisk together the flour, powdered sugar, baking powder, and salt.
Add the cold, cubed butter to the dry ingredients and use your fingertips or a pastry blender to cut it in until the mixture resembles coarse crumbs.
Add the egg yolks and vanilla sugar, then quickly knead the dough until it comes together into a smooth ball.
Wrap the dough in plastic wrap and refrigerate for at least 30 minutes to make it easier to handle.
On a lightly floured surface, roll out the chilled dough to a thickness of about 3-4 mm. Use a linzer cookie cutter to cut out circles.
Place the cookies on a baking sheet lined with parchment paper and bake in a preheated oven at 350°F (180°C) for about 10-12 minutes, or until golden brown.
Remove from the oven and let them cool completely. Then, prepare the chocolate-banana filling.
Mash the banana with a fork. Combine the mashed banana with the finely chopped dark chocolate and warm heavy cream, stirring until smooth and creamy.
Spread the chocolate-banana cream on one half of the linzer cookies, then top with another cookie.
Dust the finished linzer cookies with powdered sugar and let them sit for at least 30 minutes before serving to allow the flavors to meld.
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