In 2-quart saucepan, mix brown sugar, butter and milk. Heat to boiling over medium heat, stirring constantly with whisk. Boil 1 minute. Remove from heat; stir in vanilla. Gradually stir in powdered sugar until smooth. Stir gently 2 to 3 minutes or until slightly thickened. Immediately spoon glaze over top of cake, allowing some to drizzle down side. Top with remaining 1/2 cup pecans.