Grease two 6 inch round cake pans and line bottoms with parchment. Grease again and dust with flour.
Stir together flour, baking soda, salt and cocoa powder. Set aside.
In a large measuring cup or bowl, whisk together buttermilk, egg, vanilla and vinegar.
In a mixing bowl using an electric mixer, beat butter, oil and sugar at medium-high speed, scraping bowl, until light and fluffy. Reduce mixer speed to low and gradually add the egg/milk mixture. Mixture will most likely curdle, but it will smooth out after you add the flour mixture. Carefully add your red food coloring. Use enough so that you have a vibrant red.
Add flour mixture gradually, scraping and stirring by hand and then mixing on low until batter is smooth. Divide batter between the two pans. It will be thick.
Spread evenly in pans and bake for about 23 minutes or until a toothpick inserted in center comes out clean. Cool in pans on wire rack 10 minutes. Invert onto a cooling rack and let cool completely.
To make the frosting, beat butter in a large bowl with electric mixer on medium speed until creamy. Beat in the softened cream cheese and 1 cup of the powdered sugar until blended. Gradually add remaining powdered sugar until smooth (taste test after 2 cups, and add more as needed). Beat in vanilla.
Frost layers and stack. If you like the look of crumbs on the side of your cake, you can level off the bottom layer (cut off the dome), then crumble it up and press against the size. This is a pretty moist cake so the crumbs will be kind of moist and prone to looking tacky, but if you are good at decorating you can make it work.