01 - Place potatoes in a large pot and cover with cold, salted water. Bring to a boil, reduce heat, and simmer for 15-20 minutes until fork-tender.
02 - Melt 2 tablespoons butter in a skillet over medium heat. Add shredded cabbage and sauté for 5-7 minutes until softened but not browned. Stir in scallions and cook for another 1-2 minutes. Remove from heat.
03 - Drain potatoes well and return to the pot. Allow them to sit for 1 minute to steam dry.
04 - Mash potatoes thoroughly using a masher or ricer.
05 - Gently heat milk and remaining butter in a small saucepan until butter is melted.
06 - Pour warm milk mixture into potatoes and mash until smooth and creamy.
07 - Fold in sautéed cabbage and scallions. Season generously with salt and pepper.
08 - Transfer to a serving bowl and top with extra knobs of butter, allowing them to melt over the hot mash.