Preheat your oven to 425°F (220°C).
Peel the sweet potatoes and cut them into quarters for even cooking.
Add the sweet potatoes to a large pot of salted water and bring to a boil. Cook for 15-20 minutes, or until the potatoes are fork-tender.
Drain the sweet potatoes in a colander and let them cool slightly.
On a baking sheet lined with parchment paper, place the cooked sweet potatoes. Using a potato masher or a fork, gently smash each potato to about ½ inch thick.
In a small saucepan, melt the butter over low heat. Add the minced garlic and sauté for 1-2 minutes or until fragrant, being careful not to burn the garlic.
Drizzle the garlic butter evenly over the smashed sweet potatoes. Season with salt and black pepper to taste.
Sprinkle the grated Parmesan cheese over the potatoes.
Place the baking sheet in the oven and bake for 10-15 minutes, or until the cheese is melted and the edges of the potatoes are slightly crispy.
Remove from the oven, garnish with fresh parsley or chives if desired, and serve warm.