In a large bowl, whisk together flour and salt. Cut in cold butter until mixture resembles coarse crumbs. In a separate bowl, whisk egg, water, and vinegar. Gradually stir into flour mixture. Knead lightly, form into a disk, wrap in plastic, and chill for 30 minutes.

While the dough is chilling, heat olive oil in a skillet over medium heat. Add chopped onion and red bell pepper, sauté until softened, about 5-7 minutes. Stir in minced garlic, cumin, smoked paprika, and oregano, cooking for an additional minute. Add cooked chicken and tomato sauce, seasoning with salt and pepper to taste. Let the filling cool completely.

Preheat oven to 375°F (190°C). Roll out dough to 1/8-inch thickness. Cut out circles of approximately 4-5 inches in diameter. Place 2 tablespoons of filling in the center of each circle. Fold dough over and seal edges, crimping with a fork.

Arrange filled empanadas on a lined baking sheet. Beat extra egg and brush tops for a glossy finish. Bake for 25-30 minutes until golden brown. Allow to cool slightly before serving.