Knead butter, curd, flour and 1 pinch of salt into a smooth dough. Roll out the dough to a thickness of approx. 3 mm and cut into squares (7 x 7 cm).
Place 1 teaspoon of jam in the center of each square, fold the squares together and shape into a croissant.
Brush with egg yolk and bake the croissants at 180 °C convection oven until golden brown (approx. 10-15 minutes, watch!).
Sprinkle with sugar while still warm.