Dice sweet potatoes into 1" pieces. Transfer sweet potato pieces into a microwave safe bowl and microwave for 2 minutes. Add olive oil to the bowl and use a spoon to coat the sweet potatoes. Add brown sugar, cumin, salt, pepper, cinnamon, and cayenne (if using). Combine well. Spray a piece of foil with nonstick spray. Pour seasoned sweet potatoes onto the foil. Cut butter into squares and place on top of sweet potatoes. Close foil packets completely. Double wrap is recommended. Grill for 20 minutes, turning after 10 minutes. Serve warm.