Step 1: Preheat your oven to 400°F (200°C). Wash the sweet potatoes thoroughly and pat them dry. Prick each sweet potato several times with a fork.
Step 2: Lightly rub each sweet potato with a little olive oil and sprinkle with a pinch of salt. Place them directly on the oven rack or on a baking sheet lined with parchment paper. Bake for 45-60 minutes, or until they are very tender when squeezed.
Step 3: While the sweet potatoes are baking, prepare the filling. Heat 1 tablespoon of olive oil in a large skillet over medium heat. Add the diced red onion and bell pepper and sauté for 5-7 minutes, until softened.
Step 4: Add the rinsed black beans, corn, chili powder, cumin, smoked paprika, a pinch of salt, and a grind of black pepper to the skillet. Stir well to combine all the flavors. Cook for another 5-7 minutes, stirring occasionally, until the mixture is heated through and fragrant. Remove from heat.
Step 5: Once the sweet potatoes are baked and cool enough to handle, carefully slice each sweet potato lengthwise down the middle, being careful not to cut all the way through the bottom skin. Fluff the flesh with a fork, leaving about a quarter-inch border attached to the skin. Scoop out the majority of the soft sweet potato flesh into a medium bowl, leaving the skin intact to form a 'boat'.
Step 6: To the scooped sweet potato flesh, add the butter (if using) and milk, and season with salt and pepper. Mash lightly with a fork until mostly smooth but still a little chunky.
Step 7: Combine the mashed sweet potato flesh with the black bean and corn mixture. Stir gently until everything is well incorporated. If desired, add half of the shredded cheddar cheese into this mixture. Taste and adjust seasonings as needed.
Step 8: Spoon the savory filling generously back into each sweet potato skin 'boat'. Pile it high! Sprinkle the remaining shredded cheddar cheese over the top of each stuffed sweet potato.
Step 9: Return the stuffed sweet potatoes to the oven and bake for another 10-15 minutes, or until the cheese is melted and bubbly and the filling is heated through. For a golden-brown top, you can broil for the last 1-2 minutes, watching carefully.
Step 10: Carefully remove the stuffed sweet potatoes from the oven. Garnish with fresh chopped cilantro and a dollop of sour cream or Greek yogurt, if desired. Serve hot and enjoy!