Thoroughly wash the potatoes and remove any bad spots. Boil potatoes in a large pot of water for 20 minutes, until fork-tender. Drain and let cool slightly.
Preheat the oven to 450°F. Brush two large baking sheets with olive oil. Spread the potatoes on the baking sheets, allowing space between each potaot.
Using the flat bottom of a sturdy glass, smash the potatoes until they're ½ inch thick. If the glass is sticking to your potatoes, spray it with cooking spray or brush with olive oil.
Melt the butter, then stir in the minced garlic and chopped rosemary. Generously brush the potatoes with the butter mixture, then sprinkel with salt and pepper.
Transfer to the oven and bake for 25-30 minutes until the edges are crispy (you do not need to flip the potatoes). Remove from the oven and serve immediately.