Preheat the oven to 425°F. Boil the potatoes in salted water to cover (about 1 inch above the potatoes) for 12 to 15 minutes or until a knife can easily pierce through. Drain and let dry.
Working one at a time, place a potato on an oiled rimmed sheet pan. Place a glass over the potato and smash until about a ¼-inch thickness. Repeat with the remaining potatoes, using two baking sheets if needed and making sure there's ample room between each potato so that they're arranged in a single layer.
Stir together the melted butter and garlic in a small bowl. Brush half of the mixture over the tops of the potatoes and sprinkle lightly with salt and pepper. Bake for 20 minutes.
Flip the potatoes over and brush with the remaining garlic butter. Bake for another 10 to 15 minutes or until browned and crispy.
Remove the potatoes to a serving platter and sprinkle with chives. Serve with sour cream if desired.