Prepare the filling by mixing butter, sugar, vanilla sugar, coarsely ground cardamom, and optional cornstarch until smooth and evenly combined.
Remove the dough from the fridge and roll it out into a rectangle about 3-4 mm thick.
Spread the cardamom filling evenly over the dough.
Fold one edge toward the center, then fold the other edge over to meet the new bottom edge.
Roll the dough both vertically and horizontally to make it as high as possible, making it easier to twist the buns.
Cut the dough into strips and twist them into buns. Place the buns on a parchment-lined baking sheet.
Let the buns rise under a kitchen towel until nearly doubled in size, about 1-1.5 hours.
Brush the risen buns with egg wash (1 egg, 2 tbsp milk, and a pinch of salt).
Bake in the middle of the oven at 225°C (437°F) for about 8-12 minutes, depending on your oven and the size of the buns.
Once baked, brush the buns immediately with the syrup (200 ml water and 100 g sugar) while they are still warm.
Let the buns cool under a kitchen towel.
Dust with powdered sugar after they’ve cooled or just before serving.