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.