To make fudge layer, place butter in medium microwavable bowl. Microwave uncovered on High 45 to 60 seconds or until melted. Add brown sugar, cocoa and milk; stir with whisk until well blended. Microwave 1 1/2 to 2 1/2 minutes or until mixture comes to a boil, stirring every 30 seconds, until smooth. Add powdered sugar and vanilla; mix well.