01 - Combine flour and salt in a large bowl. Cut in cold, diced butter until mixture resembles coarse crumbs.
02 - Whisk egg, cold water, and vinegar in a small bowl. Pour into flour mixture and mix to form dough.
03 - Transfer dough to floured surface, knead briefly, shape into a disk, wrap, and refrigerate for minimum 30 minutes.
04 - Boil elbow macaroni in salted water according to package instructions. Drain and set aside.
05 - Melt butter in saucepan over medium heat. Whisk in flour, cooking for 1 minute until smooth.
06 - Gradually whisk in whole milk until slightly thickened. Stir in cheddar and mozzarella until smooth and incorporated.
07 - Season with salt and ground white pepper. Fold in cooked macaroni and allow to cool slightly.
08 - Heat olive oil in skillet over medium heat. Sauté diced onion until translucent.
09 - Add chopped brisket and barbecue sauce if desired. Cook 2–3 minutes, season with salt and black pepper. Remove from heat.
10 - On floured surface, roll dough to 1/8-inch thickness. Cut into 5-inch rounds using a cutter.
11 - Spoon brisket and mac n cheese into the center of each dough circle.
12 - Fold dough over to form half-moons. Pinch edges and crimp with fork to seal.
13 - Place sealed empanadas on parchment-lined baking sheet.
14 - Whisk egg with water. Brush over each empanada to promote browning.
15 - Bake in preheated 400°F oven for 20–25 minutes, until golden brown.
16 - Allow empanadas to cool slightly before serving hot.