Boil the potatoes: Boil fingerling or baby potatoes until they're just fork tender.
Season: Toss the boiled potatoes with olive oil, rosemary (optional), salt, and black pepper.
Prep for roasting: Place the seasoned potatoes on an oiled sheet pan.
Smash the potatoes: Smash the potatoes gently to flatten them.
First roast: Roast in an oven preheated to 400°F for 20 minutes.
Add butter and garlic: Remove the potatoes, flip them over, and add butter and minced garlic.
Second roast: Return to the oven and roast for another 20 minutes or until golden brown and crunchy.