For the dough, dilute the yeast in 1 cup milk warmed to 37-38 °C and set aside for 10 minutes. When the mixture begins to foam, mix with the remaining warmed milk, add salt, sugar, cardamom and melted butter. Gradually pour in the flour, stirring, and knead soft non-sticky dough. Put the dough in a clean bowl, cover with a film and let it rise in a warm place for 1 hour. For the filling, melt butter, mix with sugar and cinnamon. Roll out the raised dough into a 20x50 cm rectangle on a floured surface. Evenly coat the rectangle surface with a filling. Tightly roll the rectangle. Then, cut the roll into 1.5 cm thick rings with a sharp knife, often moistening it in hot water. Place the buns on a baking sheet (or 2 trays), covered with a baking parchment, at a distance of at least 2 cm from each other. Cover and let rise for 30 minutes. Meanwhile, preheat the oven to 225 °C. Brush the raised buns with an egg whipped with 1 tbsp. water and heavily sprinkle with piping. Bake at 225 °C for 10 minutes, then lessen the oven temperature to 200 °C and bake for another 10-13 minutes. The buns should be golden brown and well-baked inside. Slightly cool the buns on a grill before serving. They can be served both hot and completely cooled.