Preheat oven to 450 degrees.

Place potatoes in a large pot and cover with water. Salt the water generously. Bring to a boil and cook until potatoes are fork tender, about 20 minutes; drain and cool slightly.

In a small bowl, combine melted butter, garlic and rosemary. Drizzle a large sheet pan with olive oil.

Arrange potatoes on the pan, leaving about an inch of space between potatoes. With a potato masher or fork, press down on each potato until it flattens, but is still held together by the skin.

Drizzle the butter mixture over the potatoes. Sprinkle with additional rosemary, if desired. Season with salt and pepper.

Bake for 20-25 minutes, or until potatoes are golden brown.