In a 4- to 5-qt. Dutch oven, cook potatoes, covered, in enough lightly salted boiling water to cover 20 to 25 minutes or until tender; drain. Return the hot, drained potatoes to the hot Dutch oven. Add the 1/4 cup butter. Let stand, uncovered, 2 to 3 minutes. Meanwhile, in a small saucepan, heat the milk over low heat until hot but not boiling.
Mash potatoeswith a potato masher or beat with a mixer on low speed just until light and fluffy. Stir in the warm milk, blue cheese, salt and pepper. Gradually stir in additional milk to make potatoes desired creaminess. Sprinkle with crumbled bacon and sliced green onion. If desired, serve drizzled with additional butter.