Preheat the oven to 355 °F (180 °C).
Thaw the dough squares. If you're not using using butter puff pastry, brush the squares with melted butter/oil.
Mix the filling ingredients except for the mozzarella balls.
Line up a sheet pan with parchment paper and place the dough squares on top.
Place a spoonful of the filling and a mozzarella ball in the center of each square, brush with an egg around the edges and close as shown in the videos. Tighten the closings well and make indentations in the corners of the burek.
Brush the burek sandwiches with an egg, sprinkle with sesame seeds or black cumin and bake for about 45 minutes, or until golden.