01 - In a large mixing bowl, combine flour and salt. Add in cold diced butter and blend using a pastry blender or fingertips until the mixture forms coarse crumbs.
02 - Whisk egg, water, and vinegar together in a small bowl. Pour into flour mix and stir just until a shaggy dough develops.
03 - Transfer dough onto a lightly floured work surface. Briefly knead to smooth, shape into a flattened disk, wrap in plastic, and chill for at least 30 minutes.
04 - Cook elbow macaroni in boiling salted water according to package instructions. Drain thoroughly and set aside.
05 - In a saucepan over medium heat, melt butter and whisk in flour. Cook for 1 minute before gradually pouring in the milk. Cook, stirring, until slightly thickened.
06 - Incorporate cheddar and mozzarella, stirring until melted. Season with salt and white pepper, then fold in drained macaroni. Let cool slightly.
07 - Heat olive oil in a skillet over medium. Sauté onion until translucent. Add chopped brisket and barbecue sauce, cook for 2–3 minutes until warmed through, and season to taste.
08 - Preheat oven to 400°F. Roll chilled dough on a floured surface to 1/8-inch thickness. Cut rounds using a 5-inch pastry cutter.
09 - Spoon brisket mixture and mac n cheese into center of each dough circle. Fold over to create half-moon shapes, pinching edges to seal and crimping with a fork.
10 - Transfer assembled empanadas to a parchment-lined baking sheet.
11 - Whisk egg and water together. Brush mixture over each empanada. Bake for 20–25 minutes until golden brown.
12 - Allow empanadas to cool slightly before serving.