Method:

• Heat oven to 350°F (180°C) for shiny metal or glass pans or 325°F (160°C) for dark or non-stick pans.
• Lightly grease or spray with non-stick cooking spray, bottom and sides of two 9-inch (23 cm) round baking pans.
• In large bowl, beat cake mix, sour cream, water, oil, eggs and vanilla with electric mixer on low speed 30 seconds; beat on medium speed 2 minutes. Pour into pans.
• Bake 29 minutes or until toothpick inserted in centre comes out clean. Cool 10 minutes.
• Remove from pans to cooling rack. Cool completely, about 1 hour.
• To make frosting: In large bowl combine butter and cocoa until smooth. Slowly add icing sugar alternately with sour cream, beating with electric mixer on low speed until smooth. Stir in vanilla. Increase speed to medium and beat two more minutes until light and fluffy.
• Place one layer top-side-down on serving plate. Spread 1/2 cup (125 mL) frosting over cake layer to within 1/4-inch (6 mm) of edge. Place second layer top-side-up. Frost sides and top of cake with remaining frosting.