Serve two empanadas for a hearty vegetarian main course, or make smaller ones to serve at a cocktail party.
- 1In medium fry pan over medium-high heat, heat 1 tablespoon of the vegetable oil. Add chopped plantain and sauté until golden and slightly softened, about 5 minutes. Transfer plantains to a bowl and set aside.
- 2Heat remaining 1 tablespoon oil in the sauté pan and add onion. Sauté until golden, about 4 minutes. Add beans, cilantro, cumin, and cayenne and sauté until mixture is heated and flavors are blended, about 5 minutes. Add lime juice and season with salt. Remove from heat.
- 3Using a potato masher, mash bean mixture until it forms a coarse paste. Transfer to a bowl and set aside to cool. Add plantains to bean mixture and stir to combine.
- 4Line two baking sheets with parchment paper. In a small bowl, whisk together egg and milk to make egg wash.
- 5Spread one sheet of puff pastry on a lightly floured surface. Dust pastry lightly with flour and, using a rolling pin, roll into a 14-inch square.
- 6Using a sharp paring knife, trim dough edges. Cut dough into four equal squares. Using a pastry brush, brush edges of each square lightly with egg wash. Place 1/4 cup of cooled filling on each square and sprinkle with 2 tablespoons of queso fresco and 1 1/2 teaspoons of butter. Fold dough in half to form a triangle. Seal edges by pressing them together with the tines of a fork. Transfer to a baking sheet. Poke a few holes in the top of each empanada with the tip of a paring knife. Repeat with remaining sheet of puff pastry and remaining filling.
- 7Cover the baking sheets with plastic wrap and refrigerate for 15 to 30 minutes to firm slightly. Meanwhile, heat the oven to 400°F.
- 8Remove empanadas from the refrigerator and remove plastic wrap. Brush top of each empanada with egg wash.
- 9Bake until golden brown and flaky, 20 minutes. Transfer to a wire cooling rack to cool slightly. Serve warm, with tomatillo salsa or sour cream on the side.