Heat oven to 350°F. Grease and flour 13x9-inch baking pan.**
Beat butter and sugar in large bowl; add eggs and vanilla, beating well. Stir together buttermilk and food color. Stir together flour, cocoa and salt; add alternately to butter mixture with buttermilk mixture, mixing well. Stir in baking soda and vinegar. Pour into prepared pan.
Bake 30 to 35 minutes or until wooden pick inserted in center comes out clean. Cool completely in pan on wire rack. Frost; garnish with chocolate chips, if desired. Makes 12 servings.
* To sour milk: Use 1 tablespoon white vinegar plus milk to equal 1 cup.