01 - Place cubed potatoes in a large pot and cover with salted water. Bring to a boil over high heat, then reduce to medium and cook until fork-tender, about 15 minutes. Drain thoroughly and return to the warm pot.
02 - Add butter and warmed milk to the drained potatoes. Mash using a potato masher or ricer until smooth and creamy. Set aside.
03 - Heat a skillet over medium heat. Add shredded cabbage and sauté until tender, approximately 4 minutes. Add sliced scallions and continue cooking for 1 minute until fragrant.
04 - Stir the sautéed cabbage and scallions into the mashed potatoes. Season with salt and pepper. Spread mixture on a baking sheet and refrigerate for 15-20 minutes until cool enough to handle.
05 - Scoop approximately 2 tablespoons of cooled colcannon mixture and flatten into a disc in your palm. Place one cheddar cube in the center and wrap the potato mixture around it, sealing completely. Roll into a smooth ball. Repeat with remaining mixture to make 16 balls.
06 - Arrange three shallow bowls: one with flour, one with beaten eggs, and one with panko breadcrumbs.
07 - Roll each colcannon ball in flour, shaking off excess. Dip into beaten egg, allowing excess to drip off, then coat thoroughly with panko breadcrumbs. Place on a parchment-lined baking sheet.
08 - Pour vegetable oil into a deep skillet or Dutch oven to a depth of 2 inches. Heat to 350°F using a deep-fry thermometer for accuracy.
09 - Fry the balls in batches of 4-5 for 2-3 minutes, turning occasionally, until golden brown and crispy. Use a slotted spoon to transfer to paper towels to drain.
10 - Serve hot while the cheese center remains melted and gooey. Accompany with sour cream, spicy mustard, or garlic aioli for dipping.