Combine sour cream, garlic powder, fresh chives and salt and pepper to taste for a simple dipping sauce. Cover and refrigerate until the croquettes are ready to serve.
Crisp bacon slices in a skillet over medium heat. Reserve 1 tablespoon of the bacon grease in the skillet. Remove the bacon to a paper towel to drain and cool. Then, crumble the bacon and add them to a mixing bowl.
Add the leeks to the skillet with a little salt + pepper and cook for about 4 to 5 minutes until soft and just beginning to color. Add the leeks to the mixing bowl with the crumbled bacon.
Add the cold mashed potatoes to the bowl and combine with the bacon and leeks. Taste and season with salt + pepper as needed.
Using a small cookie scoop (about 2 tablespoon volume), scoop out a ball of potatoes. Flatten with clean hands then wrap it around a cube of Irish cheddar, turning it back into a ball. Repeat until all the potatoes are used.
Create a 3-bowl dredging station: one with flour, one with the egg (whisked up with a fork), one with the Panko.
Coat each potato ball in flour, then egg, then Panko and set aside on a sheet pan. (If you're making these ahead, cover and chill the potato balls until you're ready to cook).
Air Fryer Instructions: Preheat the air fryer at 400°F. Spritz the croquettes with cooking spray, then place them in a single layer in the air fryer basket. Cook for 6 to 8 minutes. Flip them halfway through. Repeat until all croquettes have been cooked.
Oven Instructions: Preheat oven to 425°F. Spritz the croquettes with cooking spray. Bake on a sheet pan for 10 minutes. Flip them halfway through.
The croquettes will be soft after cooking (using either method). Let them sit for a minute or two before transferring to a serving dish. Season with some coarse salt and garnish with fresh herbs, if you like.