Heat oven to 177°C (163°Cfor dark or nonstick pans). Grease and flour two 23 cmround pans, or spray with baking spray with flour.
In 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.
Bake and cool as directed on cake mix box.
To make icing, in heavy 1.9L 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.