Heat oven to 350°F. Grease two 8 in. round baking pans with Wilton Cake Release or vegetable pan spray. In large mixer bowl, beat butter, sugar and vanilla until creamy. Add eggs and icing color; blend well. Stir together flour, cocoa and salt; add alternately with buttermilk to batter mixture, beating until blended. Stir baking soda into vinegar; fold carefully into batter (do not beat). Pour batter into prepared pans. Bake 30-35 minutes or until toothpick inserted in center comes out clean. Cool 10 minutes; remove from pans to wire rack. Cool completely.