Mix 1 cup of cold mashed potatoes, ¼ cup of crumbled feta, ¼ cup of shredded kashkaval, and 1 large egg with a fork. Season with salt and pepper.
Cut 2 sheets of thawed puff pastry into 9 squares each. Put a heaping tablespoon of filling on each square, fold it into a triangle, and press the edges to seal.
Arrange on baking sheets and brush with 1 beaten egg yolk. Bake on the upper and lower racks for 35 minutes, swapping the sheets halfway, until golden brown.
