01 - Combine flour and salt in a mixing bowl. Cut cold butter into the mixture until a coarse crumb texture forms. Whisk together egg, cold water, and vinegar separately, then add to flour mixture. Mix just until dough forms, turn onto a floured surface, knead briefly, flatten into a disk, wrap, and refrigerate for 30 minutes.
02 - Boil elbow macaroni in salted water according to package directions; drain and set aside. Melt butter in a saucepan over medium heat, whisk in flour, and cook for 1 minute. Gradually add milk, whisking constantly until thickened. Incorporate cheddar and mozzarella cheeses, stirring until melted and smooth. Season with salt and ground white pepper, then fold in cooked macaroni and let cool slightly.
03 - Heat olive oil in a skillet over medium heat. Sauté diced onion until translucent. Add chopped smoked brisket and barbecue sauce if desired, cook for 2 to 3 minutes until heated through. Season with salt and pepper, then remove from heat.
04 - Preheat oven to 400°F. Line a baking sheet with parchment paper. On a floured surface, roll out dough to 1/8-inch thickness. Cut dough into 5-inch rounds. Place a spoonful of brisket and a spoonful of mac and cheese in the center of each round. Fold dough over filling to form a half-moon, pinch and crimp edges with a fork to seal. Arrange empanadas on the prepared baking sheet.
05 - Whisk egg with water to create an egg wash and brush each empanada. Bake for 20 to 25 minutes until golden brown. Allow empanadas to cool slightly before serving.