1. Cook butter in a 2-qt. heavy saucepan over medium heat, stirring constantly, 6 to 8 minutes or just until butter begins to turn golden brown. Immediately remove pan from heat, and pour butter into a small bowl. (Butter will continue to darken if left in saucepan.) Remove and reserve 1 to 2 Tbsp. browned butter.
2. Bring potatoes, 2 tsp. salt, and water to cover to a boil in a large Dutch oven over medium-high heat; boil 20 minutes or until tender. Drain. Reduce heat to low. Return potatoes to Dutch oven, and cook, stirring occasionally, 3 to 5 minutes or until potatoes are dry.
3. Mash potatoes with a potato masher to desired consistency. Stir in remaining browned butter, buttermilk, milk, pepper, and remaining 1 tsp. salt, stirring just until blended.
4. Transfer to a serving dish. Drizzle with reserved 1 to 2 Tbsp. browned butter. Garnish, if desired.
Note: To make ahead, prepare recipe as directed through Step 3. Place in a lightly greased 2 1/2-qt. ovenproof serving dish; cover and chill up to 2 days. Let stand at room temperature 30 minutes. Bake, uncovered, at 350° for 35 to 40 minutes or until thoroughly heated. Drizzle with reserved brown butter, and garnish, if desired.