Preheat a grill to medium-high and lightly oil the grates. Preheat the oven to 400 degrees F. In a small pot, melt the butter with the garlic and Sichuan peppercorns over low heat. Stir and cook for 2 minutes. Sprinkle over the brown sugar, and cook for an additional 1 to 2 minutes, stir and remove from the heat. Toss the sweet potatoes in 1 tablespoon vegetable oil, and season with salt, to taste. Grill the sweet potato slices on both sides, achieving grill marks. Transfer the grilled rounds to a baking dish and spoon the garlic butter mixture over the sweet potatoes. Roast the potatoes for 20 to 25 minutes, or until they are tender and cooked through.