Mix the base. Combine cooled mashed potatoes, parmesan, half the mozzarella, minced garlic, egg, herbs, garlic powder, salt, and pepper in a large bowl. Mix until evenly combined. Refrigerate for 15 minutes if the mixture feels soft.
Shape the balls. Scoop about 2 tablespoons of mixture, flatten slightly, place a small pinch of mozzarella in the center, then fold edges up and roll into a smooth ball. Repeat with remaining mixture.
Coat in breadcrumbs. Roll each ball firmly in panko breadcrumbs, pressing to adhere all around. Chill on a tray for 10 minutes before baking.
Bake. Preheat oven to 400°F (200°C). Arrange balls on a parchment-lined baking sheet, drizzle or spray with olive oil, and bake for 20 to 22 minutes. Flip once halfway through. Broil for 2 to 3 minutes at the end for deeper color.
Rest and serve. Transfer to a wire rack for 3 to 4 minutes before serving. Serve with your choice of dipping sauce.
