• In a large bowl, mix flour, baking powder, and salt.  Mix butter into dry ingredients with your hands  until pea sized pieces form. Add the egg and water and mix with a wooden spoon until a dough forms.  Turn out dough onto a lightly floured surface. Knead until smooth, for about 5 minutes. Wrap in plastic wrap and refrigerate for at least 1 hour.
  • Preheat the oven to 400°. Line 2 large baking sheets with parchment paper. In a large skillet over medium heat, heat oil.  Add onion and cook, stirring, until softened, for about 7 minutes. Add garlic and cook.  Stir until fragrant, about 1 minute longer. Add beef and cook, breaking up the meat with a wooden spoon, until no longer pink, which will take about 5 minutes. Drain fat.
  • Return pan to medium heat, and stir tomato paste into beef. Add oregano, cumin, and paprika.  Season with salt and pepper. Add jalapeños and tomatoes.  Cook, stirring occasionally, until warmed through, about 3 minutes. Let cool slightly.
  • Lightly moisten outer edge of a dough round with water. Place about 2 tablespoons filling in the center. Top with cheese. Fold dough in half over filling. Using a fork, crimp edges together. Repeat with remaining filling and dough.
  • In a small bowl, beat the egg with a bit of water. Place the empanadas on prepared baking sheets and brush with egg wash.  Bake empanadas until golden brown and filling is warmed through, about 25 minutes. Top with cilantro and serve with sour cream alongside.