Preheat oven to 425 degrees F. Bring a pot of salted water to a boil. Once boiling, add in petite potatoes and cook for 15 minutes, or until just fork tender.
Drain potatoes and add to a large bowl with 2 tablespoons olive oil, rosemary, minced garlic, kosher salt and black pepper. Toss to combine and evenly coat the potatoes.
Dump the coated potatoes onto a large baking sheet lined with parchment paper. Evenly distribute the potatoes.
Wrap the bottom of a flat-bottomed glass with parchment paper and smash the potatoes. Note: I use the rim of my baking sheet as a guide for how much to smash the potatoes, so they stay together.
Drizzle the smashed potatoes with 1 tablespoon of olive oil. Sprinkle each with a few shreds of Parmesan cheese. Bake in the preheated oven for 18 to 20 minutes.
Remove pan from the oven and use a thin spatula to carefully flip the potatoes. Drizzle with 1 tablespoon olive oil and return to the oven for 20 minutes longer, or until crispy.
Serve with ranch dressing or pesto as a dipping sauce.