In a small bowl, cream butter and sugar until light and fluffy. Beat in the sour cream, egg, food coloring, vinegar and vanilla. Combine the flour, cocoa, baking soda and salt; add to creamed mixture alternately with buttermilk.
Pour into three 5x3x2-in. loaf pans coated with cooking spray. Bake at 350° for 14-18 minutes or until a toothpick inserted in the center comes out clean. Cool for 10 minutes before removing from pans to wire racks to cool completely.
For frosting, in a small saucepan, whisk flour and milk until smooth. Bring to a boil; cook and stir for 2 minutes or until thickened. Cover; cool to room temperature.
In a small bowl, cream butter and sugar. Beat in vanilla and milk mixture until fluffy. Spread frosting between layers and over top and sides of cake.