01 - Combine flour and salt in a large bowl. Add cold cubed butter and cut in using a pastry blender or fingertips until mixture resembles coarse crumbs.
02 - Whisk egg, cold water, and vinegar in a small bowl. Add to flour mixture and stir until dough forms. Knead gently, shape into a disk, wrap in plastic wrap, and refrigerate for 30 minutes.
03 - Heat oven to 375°F and line a baking sheet with parchment paper.
04 - In a bowl, combine mozzarella, feta or queso fresco, parmesan, parsley, and black pepper.
05 - On a floured surface, roll dough to 1/8-inch thickness. Cut into 4- to 5-inch diameter circles.
06 - Place 2 tablespoons of filling in the center of each dough circle. Fold over to create half-moon shapes. Seal edges by pressing and crimping with a fork.
07 - Arrange empanadas on prepared baking sheet and brush tops with beaten egg.
08 - Bake empanadas for 18 to 22 minutes until golden brown. Allow to cool slightly before serving.