Take the puff pastry dough to melt. If you're in a hurry, it will soften quickly under hot water - just don’t open the bag!
Roll the puff pastry dough sheet into a slightly thinner and larger rectangle. Place it on a baking tray on a baking sheet. Using a sharp knife, slice the butter dough so that it is 12 squares. The knife doesn't even need to go through the dough. Brush the buttered dough with beaten egg.
Place a spoonful of jam in the centre of each square. Sprinkle the caster sugar over the entire sheet and place it in the oven.
Bake quickly, about 10-15 minutes on the middle rack of the oven. They will rise into shape like magic.
Allow the pastries to cool for a moment before serving.