Method1) Fire up. Prepare a charcoal grill for indirect cooking by placing a chimney full of lit charcoal briquets on one side of the grill's charcoal grate in order to create direct and indirect cooking zones. Adjust the smoker or grill vents to bring the temperature to about 425°F. On a gas grill, adjust the temperature knobs so that one half of the grill is off and the other half is heated enough to maintain a temperature of approximately 425°F on the indirect side.2) Prep. Place a sweet potato between two wooden spoon handles or chopsticks to create an even stopping point about 1/8-inch from your work surface. Make vertical cuts 1/4-inch apart all across the top of the sweet potato, stopping at the wooden spoon or chopstick approximately 1/8-inch from the bottom. Repeat with the remaining sweet potatoes.3) Cook. Brush each hasselback sweet potato with olive oil and place them on the grill’s main cooking grate as far away from the heat source as possible. Cover the grill, and allow the sweet potatoes to cook until fork tender, about 1 hour.4) Place the butter, pecans, maple syrup, salt, and rosemary in a small saucepan, and set the saucepan on the hot side of the grill. Allow the butter to melt, swirling or stirring occasionally to prevent burning.5) When the butter melts, spoon half of the mixture over the tops of the hasselback sweet potatoes, using a butter knife if needed to gently separate the layers so that the pecans can fall between them. Cover the grill and allow the sweet potatoes to cook for an additional 5 minutes.6) Serve. Remove the hasselback sweet potatoes from the grill to plates or a platter, and spoon on the remaining maple-pecan butter. Serve immediately.