Place sweet potatoes in a covered steamer basket set over boiling water. Steam for 10 minutes; set aside.
In a large skillet heat oil over medium. Add onion and bell pepper and cook, stirring occasionally, until softened, about 5 minutes. Add garlic, cumin, chili powder, and salt, Cook until fragrant, about 1 minute. Add sweet potato, black beans, and corn, and cook, stirring occasionally, until warmed through, about 3 minutes. Stir in the lime juice and ¼ cup water to moisten. Remove from heat and let cool.
Preheat oven to 400°F and line two large baking sheets with parchment paper. Place an empanada wrapper on baking sheet and spoon 2 generous tablespoons of the filling onto one half. Fold wrapper over the filling and press gently to seal. Crimp with a fork. Repeat with the remaining wrappers and filling.
In a small bowl, whisk together the egg with 1 tablespoon water. Brush tops of empanadas with the egg wash. Bake until golden and warmed through, about 18 minutes. Empanadas can be frozen and reheated in the microwave for 1 to 2 minutes. To pack, wrap in foil or place in an airtight container.
For more heat, divide the black bean and sweet potato mixture in half. Add 2 tsp. minced chipotle peppers in adobo sauce to one portion. Crimp the empanadas differently so you can remember which ones are spicy!