In a small bowl, mash the sweet potatoes with a fork, and mix in the cream cheese, brown sugar if using, lemon juice, and pinch of salt until creamy. Remove the dough from the tube and separate into 4 rectangles. Working with 1 rectangle at a time, roll over the perforated lines in the dough to seal together. Cut the rectangle in half and then cut the halves into 2 triangles. Place a heaping teaspoon of sweet potato filling in the center of each of 4 triangles. Fold the dough over on each, as if you were making ravioli, and pinch the edges together.
Continue making little triangle empanadas with the rest of the dough, 16 total. Place the triangles on a parchment-lined baking sheet and into the preheated oven. Bake the empanadas until golden, about 10 to 12 minutes. Meanwhile, in a shallow bowl, mix together the sugar, cinnamon, and pinch of salt. While still warm, brush the empanadas with melted butter, and then gently toss in the cinnamon-sugar mixture. Serve the empanadas with tangy plain yogurt for dipping.
Use a tiny bit of water if the dough is too dry to seal. Omit the brown sugar if the sweet potato leftovers are already sweetened.