01 - In a large bowl, combine flour and salt. Cut in cold butter 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 mix until dough forms. Knead gently, shape into a disk, wrap in plastic wrap, and refrigerate for 30 minutes.
03 - Preheat oven to 375°F. Line a baking sheet with parchment paper.
04 - In a bowl, mix mozzarella, feta or queso fresco, Parmesan, parsley, and black pepper until evenly incorporated.
05 - On a lightly floured surface, roll dough to 1/8-inch thickness. Cut into 4- to 5-inch circles.
06 - Place 2 tablespoons of cheese filling in the center of each circle. Fold over to form a half-moon, press edges to seal and crimp with a fork.
07 - Arrange empanadas on prepared baking sheet and brush tops with beaten egg.
08 - Bake for 18 to 22 minutes, or until empanadas are golden brown. Allow to cool slightly before serving.