Peel and chop the Yukons and place them in a large pot, cover with cold salted water, and bring to a boil. Cook until fork-tender, about 15 minutes. Drain thoroughly.

Return hot potatoes to the pot or a large bowl. Add butter, sour cream, warm milk, garlic, salt, and black pepper. Mash until smooth and fluffy.

Preheat oven to 375°F. Grease a 9x13-inch baking dish and spread half of the mashed potatoes into the dish. Sprinkle with 1 cup of cheddar cheese. Add remaining mashed potatoes and top with the final 1/2 cup of cheese.

In a small bowl, mix breadcrumbs with melted butter and sprinkle over the top of the casserole.

Bake uncovered for 25–30 minutes or until the top is golden brown and bubbly. Garnish with chopped chives or green onions if desired.