Preheat your oven to 500 degrees. Adjust your oven racks so that there is one near the top and one near the bottom.
Using a rimmed baking sheet lined with foil, place your fingerling potatoes along with 3/4 cup of water and wrap with aluminum foil creating a tight seal.
Cook on the bottom rack until easily pierced by knife, approximately 25-30 minutes.
Remove from oven, discard foil, and let cool for 15 minutes. If any water remains, discard and blot your potatoes dry.
Drizzle 3 tablespoons of oil over your potatoes and roll to coat. Using another similarly sized baking sheet or flat bottom dish, place on top of potatoes and firmly push down to flatten until approximately 1/3-1/2 inch thick.
Season with garlic salt and pepper. Drizzle the remaining 3 tablespoons of olive oil.
Roast potatoes on the top oven rack for 15 minutes. Remove from oven, flip, sprinkle with chopped rosemary and move potatoes to bottom rack to continue roasting for 15 minutes. Remove from oven, drizzle with melted butter, and place back in oven for an additional 10 minutes. Remove from oven and serve immediately.
