Put flour, baking powder, and butter cubes in a food processor. Process until a crumbly mixture is obtained.
Add sour cream and process until a uniform dough is obtained (the dough is will be very soft and sticky).
Cover the dough with plastic wrap and refrigerate for a minimum of 2 hours and up to 12.
Preheat the oven to 350°F/180°C.
Squeeze off a small piece of dough. Put a chocolate cube inside, fold it over and over to make a ball about the size of a ping pong. Then pinch the dough around to seal it completely (make sure you patch up any holes).
Place the balls on a baking sheet covered with parchment paper.
Mix the egg with 1 tablespoon of water and brush the balls.
Sprinkle some sugar.
Bake for 15-20 minutes until golden.
Serve warm or at room temperature and get ready to impress your guests.