Put lukewarm milk, sugar, yeast, flour, salt and egg whites in a bowl and mix until the ingredients combine.
Add butter in small cubes and knead to a smooth dough. (About 10 minutes)
Cover the dough and leave to rise for about one hour in a warm place.
Roll out the dough on a floured surface. Leave to rest while making the filling.
For the filling, mix all ingredients in a bowl using a whisk until combined.
Roll out the dough a little more if necessary.
Cut out squares of about 12 cm. (with a knife or pizza cutter) (I had 16 squares.)
Place in the middle of each square a tablespoon of the filling.
Moisten the corners with a bit of water.
Press the opposite corners together.
Place on a baking tray with baking paper. (I had 2 trays)
Brush with the yolk-milk-mixture.
Leave to rise for another 30 minutes.
Bake in preheated oven at 180° C for about 18 to 20 minutes.
Dust with powdered sugar.