Preheat oven to 450°F. Line sheet pan with parchment and lightly grease surface with oil or pan spray and set aside.
Add potatoes to cold, salted water and bring to a boil. Cook for 15-29 minutes, or until potatoes are fork-tender. Drain well and allow to air dry for 5-10 minutes.
Toss potatoes in cornstarch, coating evenly.
Place potatoes in single layer on sheet pan and smash each with a flat spatula so that each become approximately 1/2 inch thick.
In a medium mixing bowl, combine olive oil, melted butter. Garlic, salt and cracked black pepper, stirring until well combined. Spoon seasoned oil mixture over each smashed potato, ensuring that some oil pools slightly around each of them.
Cook potatoes in oven for 25-35 minutes, flipping halfway through. Remove from heat and hold warm for service.
