To make icing, in heavy 2-quart saucepan, heat butter over low heat, stirring constantly, until brown; remove from heat. Stir in powdered sugar, reserved cake batter and evaporated milk. Cook over medium heat, stirring constantly with whisk, until smooth, bubbly and almost thick; remove from heat. Stir in vanilla. Cool slightly. Fill layers and frost cake with icing. Store loosely covered.