Preheat the oven to 400°F (200°C). Prepare a standard 12-well muffin tin by greasing each cavity or lining with baking cups.
In a spacious mixing bowl, combine the mashed potatoes, eggs, cheddar cheese, Parmesan, sour cream (or Greek yogurt), and green onions if using. Stir until the texture is cohesive and uniform.
Incorporate the garlic powder, salt, pepper, and baking powder into the potato mixture. Mix thoroughly until the batter becomes light and well aerated.
Distribute the prepared mixture among the muffin tin wells, filling each approximately three-quarters full.
Transfer to the oven and bake for 20 to 25 minutes, or until the tops are golden and exhibit a gentle crispness.
Allow the puffs to rest for 5 minutes in the tin before removing. Serve warm, optionally accompanied by sour cream, ranch, or a preferred dipping sauce.