Step 1: Preheat your oven to 400°F (200°C). Lightly grease a large baking sheet or line it with parchment paper. For an extra crispy bottom, place the prepared baking sheet into the oven as it preheats.
Step 2: In a large mixing bowl, combine the chilled mashed potatoes, shredded sharp cheddar cheese, shredded mozzarella cheese, beaten egg, all-purpose flour, salt, black pepper, garlic powder, and optional onion powder. Mix everything together thoroughly until well combined. The mixture should be firm enough to handle and shape easily. If it feels too sticky, you can add an additional tablespoon or two of flour.
Step 3: Pour the panko breadcrumbs into a shallow dish. Using a tablespoon or a small scoop, form the potato mixture into balls, about 1 to 1.5 inches in diameter, or into small cylindrical shapes. Gently roll each potato puff in the panko breadcrumbs, ensuring it is fully coated on all sides.
Step 4: Carefully arrange the coated potato puffs on the preheated baking sheet, leaving a small space between each puff to allow for even crisping. Lightly brush the tops of the puffs with olive oil or melted butter. This helps them achieve a beautiful golden-brown color and added crispiness.
Step 5: Bake for 20-25 minutes, or until the cheese puffs are golden brown and wonderfully crispy on the outside. For even browning, you can gently flip them halfway through the baking time.
Step 6: Remove from the oven and serve your crispy mashed potato cheese puffs immediately while hot. They are delicious on their own or with your favorite dipping sauces like ranch, sour cream, or spicy ketchup. Enjoy!