Step 1: Prepare the Potato Mixture

1. In a large mixing bowl, combine the leftover mashed potatoes, egg, shredded cheddar cheese, Parmesan cheese, flour (or breadcrumbs), green onions, garlic powder, and onion powder.
2. Season with salt and black pepper to taste, and add any optional ingredients, like crumbled bacon or diced jalapeños, if desired.
3. Mix until all ingredients are well combined and you have a thick, cohesive mixture.

Step 2: Shape the Potato Puffs

1. Preheat your oven to 400°F (200°C) if baking, or heat oil in a skillet if frying.
2. Lightly grease a muffin tin with olive oil or cooking spray if baking, or prepare a baking sheet with parchment paper for a flatter, crispy version.
3. Using a spoon or cookie scoop, portion out small balls of the potato mixture, about 1-2 tablespoons each. Gently roll each portion into a ball with your hands.
4. Place each ball into the prepared muffin tin or on the baking sheet, or set aside if frying.

Step 3: Bake or Fry the Puffs

Baking Method

1. Place the muffin tin or baking sheet in the preheated oven and bake for 20-25 minutes, or until the cheese puffs are golden brown and crispy on the edges.
2. If you want extra crispy tops, turn on the broiler for the last 2-3 minutes, watching closely to prevent burning.
3. Remove from the oven and let the cheese puffs cool slightly before removing them from the muffin tin or baking sheet.

Frying Method

1. Heat about 1/4 inch of oil in a large skillet over medium heat.
2. Once the oil is hot, carefully add the potato balls, a few at a time, without overcrowding the pan.
3. Fry each puff for 3-4 minutes per side, or until golden brown and crispy. Remove and drain on a paper towel-lined plate to absorb excess oil.

Step 4: Serve and Garnish

1. Garnish the Mashed Potato Cheese Puffs with fresh parsley or chives and a sprinkle of extra shredded cheese, if desired.
2. Serve warm with dipping sauces like sour cream, ketchup, or a creamy ranch dressing for an extra treat.