In a large mixing bowl, combine the cold mashed potatoes, shredded sharp cheddar, crumbled crispy bacon, minced chives, sour cream, egg yolk, garlic powder, onion powder, smoked paprika, salt, and black pepper. Stir gently with a wooden spoon or spatula until evenly distributed.

Using a 2-tablespoon cookie scoop or measuring spoon, scoop portions of the potato mixture and shape them with clean hands into smooth cylindrical croquettes (about 2.5 inches long and 1 inch thick). Place them onto a parchment-lined baking sheet.

CRITICAL STEP: Chill the shaped croquettes in the refrigerator for at least 30 to 45 minutes (or 15 minutes in the freezer). Chilling firms the butter and cheese, preventing the croquettes from bursting open during frying.

Set up three shallow bowls in a row: Bowl 1 with the all-purpose flour; Bowl 2 with the beaten eggs and milk; Bowl 3 with the seasoned panko breadcrumbs.

Dredge each chilled croquette in the flour, tapping off any excess. Dip thoroughly in the beaten egg wash, and then roll firmly in the panko breadcrumbs, gently pressing so the flakes adhere to all sides.

Heat about 1/2 inch of oil in a large cast-iron skillet or Dutch oven over medium-high heat until it reaches 365°F (185°C). Test by dropping in a few panko crumbs; they should sizzle immediately without smoking.

Fry the croquettes in batches of 5 to 6, avoiding crowding the pan. Cook for 2 to 3 minutes per side, gently turning with tongs, until all sides are deep golden brown and exceptionally crunchy (about 4 to 5 minutes total).

Transfer with a slotted spoon to a paper towel-lined wire rack to drain. Immediately sprinkle with a pinch of coarse sea salt while sizzling hot. Repeat with remaining batches.

Arrange the warm croquettes on a platter, garnish with extra fresh chives, and serve immediately alongside the chive sour cream dip.