Arrange a rack in the middle of the oven and heat the oven to 400°F. Line a baking sheet with aluminum foil.
Place 3 tablespoons unsalted butter in a large microwave-safe bowl. Microwave on HIGH in 10-second bursts, stirring between each burst, until the butter is fully melted, 30 to 40 seconds total.
Prep the following, adding them to the bowl of melted butter: Cut 1 1/2 pounds fingerling potatoes in half lengthwise. Mince 3 cloves garlic. Add 2 tablespoons olive oil, 1 teaspoon kosher salt, and 1/2 teaspoon black pepper. Toss to combine, making sure the potatoes are evenly coated. Transfer to the baking sheet and arrange in a single layer cut-side down.
Roast until the potatoes are tender enough to easily be pierced with a fork, golden brown, and starting to shrivel slightly, flipping halfway through, 25 to 30 minutes total. Meanwhile, finely chop until you have 2 tablespoons fresh parsley leaves.
Taste and season the potatoes with more salt and pepper if needed. Garnish with the parsley.
