Boil the potatoes – Place the potatoes in a large pot and fill the pot with water until the potatoes are covered by at least 1 inch. Season the water with a generous amount of salt. Set the pot over medium-high heat and bring the water to a rolling boil. Boil the potatoes until they’re fork tender, about 15-18 minutes.
Dry the potatoes – Once the potatoes are fork tender, drain them in a colander. Let the potatoes sit for 5-10 minutes, giving the colander a shake halfway through, until the potatoes are completely dry.
Preheat the oven – Meanwhile, preheat the oven to 450°F (232°C). Grease a sheet pan with olive oil.
Season the potatoes – Spread the dried potatoes on the prepared sheet pan. Drizzle the potatoes with the olive oil. Then, sprinkle the garlic powder, onion powder, paprika, and cayenne over the top. Using a wood spoon or clean hands, toss the potatoes until they’re well-coated in the oil and seasonings.
Smash the potatoes – Using a flat-bottomed drinking glass or ramekin, gently smash the potatoes until they’re about 1/2-inch thick. If they’re sticking, coat the bottom of the glass or ramekin with some cooking spray or olive oil. Then, spread the potatoes out on the sheet pan until they’re about 1/2-1 inch apart.
Bake the potatoes – Place the sheet pan in the oven. Bake the potatoes for 20 minutes, flipping them halfway through, until they’re crispy on the outside and tender on the inside.
Make the garlic butter – Meanwhile, add the butter, garlic, and rosemary to a microwave-safe small bowl. Microwave the mixture for about 30 seconds, until the butter is completely melted. Alternatively, you can melt the garlic butter in a small saucepan over medium-low heat.
Add the garlic butter – After the potatoes have crisped up, remove the sheet pan from the oven. Spoon the garlic butter evenly over the top of each potato. Bake the potatoes for another 5-10 minutes until the garlic is fragrant and the bottoms of the potatoes are very crispy.
Serve the potatoes – Transfer the potatoes to a plate or serving platter. Sprinkle flaky salt over the potatoes, if desired. Enjoy!