Preheat the oven to 425F (220C). Line a rimmed baking sheet with parchment paper or foil for easier cleanup.
Place the cubed sweet potatoes in a large bowl. Drizzle with olive oil and toss so each piece is lightly coated.
In a small bowl, whisk together brown sugar, maple syrup, apple cider vinegar, cinnamon, cumin (if using), salt, and pepper until smooth.
Pour the sugar-spice mixture over the sweet potatoes and toss gently to coat evenly. Make sure the cubes are spread out in a single layer on the prepared baking sheet; crowding will steam them instead of caramelizing.
Roast for 25 minutes, then remove the pan and gently flip the potatoes with a spatula. Return to the oven and roast for an additional 10-15 minutes, until edges are browned and the centers are tender when pierced with a fork.
If using, brush with melted butter about 3 minutes before the end of roasting for extra gloss and richness. Keep an eye so the sugar doesn't burn.
Let the sweet potatoes rest for 5 minutes on the baking sheet. Garnish with chopped parsley or cilantro before serving.