Preheat the oven to 400 degrees F and line a baking sheet with parchment paper.
Scrub the potatoes and pat them completely dry.
Lay one potato between two chopsticks or wooden spoon handles. Make thin crosswise cuts down the length of the potato, about ¼ inch apart or less, stopping when the knife is about two-thirds of the way through. Use chopsticks as a guide, or see notes for other recommendations. Repeat with the remaining potatoes.
Melt the butter in a small saucepan over medium heat until it just starts to bubble. Turn off the heat, stir in the garlic, and let it infuse for about 1 minute.
Arrange the potatoes cut side up on the prepared baking sheet. Brush the tops with garlic butter, working it into the cuts and leaving the garlic in the saucepan. Season with the salt and pepper.
Bake on the center rack for 15 minutes. Remove, brush with garlic butter again, and return to the oven for 15 more minutes.
Brush the potatoes one last time and bake for 10 more minutes, until the edges are golden and crisp and a knife slides easily into the center.
Let the potatoes rest for a few minutes, until cool enough to handle. Arrange on a platter, sprinkle with chives, and serve with sour cream on the side to dollop the tops.