Melt butter in a medium saucepan over medium heat. Add brown sugar. Cook 3 minutes; stir constantly with a whisk. Add milk, 1 tablespoon at a time; cook 3 minutes, stirring constantly. Cool. Stir in vanilla. Combine butter mixture and powdered sugar in a bowl; beat at high speed of a mixer until smooth. Frost cake as directed; sprinkle with pecans.
Note: If it's too thick, add 1 tablespoon evaporated fat-free milk.