In a stand mixer fitted with the paddle attachment, cream butter and sugar, until light and fluffy. Meanwhile, in a small bowl, make a paste out of the red food coloring, cocoa and salt. Remove bowl from stand mixer and mix in paste by hand to the creamed butter. In a measuring cup, stir eggs with buttermilk and add to batter. Mix in vinegar and vanilla, and then add flour and baking soda. Mix until combined but do not over mix.