Heat oven to 375 degrees. Wash and dry potatoes, and prick skins with a fork in several places. Bake until potatoes are tender, about 1 hour.
Peel potatoes, and discard skin; transfer flesh to the bowl of an electric mixer fitted with the paddle attachment. Mix on medium speed until smooth, about 2 minutes. Remove, and discard any stringy fibers from paddle. Scrape sides with rubber spatula.
Combine butter and brown sugar; mix about 30 seconds. Add remaining ingredients; mix until combined. Serve immediately.