Prepare the dough: Place butter, powdered sugar, flour and Biscoff Spread in a mixer and blend into a crumbly mixture. Add the egg yolk and continue to mix until the dough is formed.
Form a ball form the dough and flatten it slightly. Wrap in plastic wrap and refrigerate for an hour at least.
Move the dough to a lightly floured surface and roll it out to a thickness of approximately 1/5 of an inch. Cut out approximately 25 circles about 3 inches in diameter with a cutter, a ring or a thin-walled glass.
Assemble the Taschen and bake: Add Biscoff Spread into a pastry bag. If it is too stiff, heat it for a few seconds beforehand in a microwave to soften it (take care not to completely liquefy it).
Pipe in the center of each circle of dough a spoonful of Biscoff Spread and sprinkle a few cookie crumbs above it. Tighten it in the form of a triangular dumpling. Make sure to tighten the edges well so that the filling will not escape during the baking process. Place it in a pan lined with baking sheet and move to a freezer for approximately 30 minutes.
Preheat an oven to 340 degrees.
Bake for approximately 15 minutes until it achieves a light golden hue and allow it to cool entirely.
You can store it in a sealed container at room temperature for up to 4 days. Serve with sprinkled powdered sugar.