Heat oven to 350°F. Place paper baking cup in each of 24 regular-size muffin cups. In small bowl, mix flour, cocoa, baking soda and salt. In large bowl, beat sugar, butter and eggs with electric mixer on medium speed until light and fluffy. Beat in food color and vanilla. On low speed, alternately add flour mixture and buttermilk just until blended. Divide batter evenly among muffin cups.