Cook sweet potatoes until soft. See Note 1 for different cooking methods.
In a shallow bowl, place sweet potatoes, peeled, and mash them with a potato masher until smooth.
Heat a tablespoon of olive oil in a skillet over medium high heat. Add chopped onions and sauté for 3 to 4 minutes, or until the onions are translucent and slightly golden around the edges.
Add minced garlic and sauté for another minute.
Stir in brown sugar, chipotle chili powder, ground cumin, salt and pepper. Cook for another 30 to 45 seconds till the spices are toasted and onions have caramelized.
Add the onion mix to the mashed sweet potatoes and mix well. Keep the filling aside.
Prepare an egg wash by whisking a large egg with a tablespoon of water.
Unfold one sheet of the cold puff pastry on a well-floured cutting board. Using a rolling pin, roll the sheet into a rough 12-inch square.
Now, using 5-inch round cookie cutter, cut 4 dough circle from 12-inch square of puff pastry. (You can also use a dessert plate as a template and a sharp paring knife to cut out 4 dough circles.)
Place two heaping tablespoons of sweet potato filling on each circle, leaving an inch border.
Brush the edges with egg wash.
Fold over the dough to create a half circle and seal and crimp the edges by pressing with a fork.
Repeat with the second sheet of puff pastry.
Preheat the oven to 375°F and line a baking sheet with parchment paper.
Arrange the empanadas on the prepared baking sheet.
Brush the tops with the egg wash and sprinkle lightly with kosher salt (optional).
Cut two tiny slits on the empanadas for the steam to escape.
Bake for 25 minutes or until the tops are golden.
Serve hot with chimichurri sauce or your favorite salsa.