Place potato cubes in a large pot of cold salted water. Bring to a boil over high heat, then reduce to a simmer and cook until just tender when pierced with a knife, about 10-12 minutes. Do not overcook - they should hold their shape. Drain well and let steam in the colander for 5 minutes to dry out.

Preheat oven to 400F. Transfer dried potatoes to a large bowl. Drizzle with olive oil, season with salt and pepper, and toss gently to coat without breaking them up.

Spread half the potatoes in an even layer in a greased 9x13 inch baking dish. Dot with half the sour cream (dolloped in small spoonfuls), drizzle with half the melted butter and milk, then sprinkle with 1 cup cheese, half the bacon, and half the scallions.

Layer the remaining potatoes on top, pressing down gently. Repeat with remaining sour cream, butter, milk, and 1/2 cup cheese (reserve remaining cheese for later).

Bake uncovered for 20 minutes until bubbling around the edges. Remove from oven, sprinkle with remaining 1/2 cup cheese, and return to oven for 10-15 minutes until cheese is melted and lightly golden in spots.

Let rest for 10 minutes to set. Top with remaining bacon and scallions. Serve hot.