Wash potatoes in tepid water using a vegetable brush. Be gentle, but use enough force to clean any dirt off the skin. Pat each potato dry with paper towels.
Take a strong metal skewer and push it through about 1/3” from the bottom of the potato lengthwise. This will act as a cutting guide since you do not want to slice the potatoes all the way through. Using a sharp knife, cut straight down until you hit the skewer. Refer to the recipe photo for more guidance.
Preheat your grill or smoker for 275 degrees F.
Coat cast-iron pan with olive oil, and place each Hasselback potato into the pan.
Melt butter and season with garlic powder, black pepper, and salt (optional). Brush half of the butter mixture onto the slits of the potato, reserving the second half for later
Place cast iron pan with the potatoes onto the grill grate, close the lid, and let cook for 1 hour. Brush with remaining butter mixture, and cook for an additional 50-60 minutes, or until the potatoes are tender and reach an internal temperature of 210 degrees F.
Once cooked, carefully remove the pan from grill and set onto a heat resistant surface. Let the potatoes sit for 10 minutes. Garnish with chopped parsley or chives, and serve.
Optional step: Crisp up the potatoes in your oven. Broil on the highest setting for 3 minutes.