Preheat your oven to 350°F. Butter and flour two six-inch cake pans and line the bottoms with parchment paper.
In a medium bowl, beat butter until light and fluffy. Add sugar and beat until well-combined, about 2 minutes.
Add egg, oil, food coloring, vinegar, and vanilla, and beat until well-combined.
In a small bowl, whisk together sifted flour, cocoa podwer, baking soda, and salt.
Add half of the flour mixture to the wet ingredients and beat until just-combined. Add all of the buttermilk, mixing until just-combined, repeating with the rest of the flour mixture. Use a rubber spatula to scrape down the edges and bottom of the bowl to make sure everything is mixed.
Divide batter evenly between prepared cake pans.
Bake for 21 to 26 minutes, until a toothpick inserted into the centers of the cakes comes out clean or with just a few crumbs.*
Cool cakes in their pans for 10 to 15 minutes before running a plastic knife or spatula around the edges of the cakes to make sure they're not stuck to the pan. Remove cakes from pans and transfer them to a cooling rack (discard parchment paper). Cool completely before frosting.