Preheat oven to 350 degrees F. Spray two 8 inch round cake pans with baking spray.
In bowl of a stand mixer or in a large mixing bowl using an electric mixer, cream butter, sugar and salt until light. Add the vanilla and the eggs beating 30 seconds after each egg is added. Make a paste of food coloring and cocoa and add to the creamed mixture. Add buttermilk and flour alternately, beating at low speed until mixed.
In a small cup, combine the soda and the vinegar and let it foam up. Pour the foaming mixture into the batter and stir until it’s mixed in. Immediately pour into the two 8 inch pans and bake for 27-30 minutes.
Let cool for 10 minutes in pans set on a rack. Flip from pans and let cool completely. Make icing.
Beat the butter and cream cheese until smooth. Beat in vanilla. Add confectioner’s sugar slowly, beating until smooth. For a little tartness, beat in some lemon juice.