1Heat oven to 350°F (325°F for dark or nonstick pans). Grease and flour two 9-inch round pans, or spray with baking spray with flour.
2In large bowl, beat cake ingredients with electric mixer on low speed 1 minute, scraping bowl constantly, then on medium speed 2 minutes. Remove 1/2 cup of batter; cover and refrigerate to use in icing. Pour remaining batter into pans.
3Bake and cool as directed on cake mix box.
4To 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.