Step 1: Preheat your oven to 400°F (200°C). Wash the sweet potatoes thoroughly and prick them several times with a fork.
Step 2: Place the sweet potatoes directly on the oven rack and bake for 45-60 minutes, or until very tender when pierced with a fork. The cooking time will vary depending on the size of the potatoes.
Step 3: Once baked, remove the sweet potatoes from the oven and let them cool for about 10-15 minutes, or until they are cool enough to handle.
Step 4: Carefully slice each sweet potato in half lengthwise. Using a spoon, gently scoop out the cooked flesh into a medium bowl, leaving a 1/4-inch border of flesh inside the skins to maintain their shape.
Step 5: To the scooped sweet potato flesh, add the melted butter, milk (or heavy cream), 1/2 cup sharp cheddar cheese, 1/4 cup mozzarella cheese, garlic powder, salt, and black pepper. Mash with a fork or potato masher until well combined and creamy. Taste and adjust seasonings as needed.
Step 6: Generously spoon the sweet potato mixture back into the empty sweet potato skins, heaping it slightly. You might have a little extra filling depending on the size of your potatoes.
Step 7: In a small bowl, combine the remaining 1/4 cup sharp cheddar and 1/4 cup mozzarella. Sprinkle this cheese mixture evenly over the top of each stuffed sweet potato half. Dust lightly with paprika.
Step 8: Return the stuffed sweet potatoes to the oven (or place them on a baking sheet if you prefer) and bake for another 10-15 minutes, or until the cheese is melted, bubbly, and lightly golden brown.
Step 9: Remove from the oven. Garnish with fresh chopped chives, if using, and serve hot. Enjoy your delicious Cheese-Stuffed Sweet Potato Bake!