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Preheat the oven to 350 degrees. Spray 3 (8-inch) round cake pans with cooking spray. Line the bottom of the pans with parchment, spray the parchment, and set pans aside.
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Stir together the flour, cocoa powder, baking soda, and salt in a small bowl. Set aside.
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Combine the sugar, oil, eggs, vanilla, and vinegar in a large bowl. Beat with an electric mixer set to medium until very smooth, about 2 minutes. Stop to scrape down the sides and bottom of the bowl as necessary.
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With mixer on low, gradually add the dry ingredients and beat until just incorporated. Scrape down the sides and bottom of the bowl.
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Again with mixer on low, beat in the buttermilk, then the food coloring. Use a rubber spatula to mix the batter a few times, taking care to scrape up from the bottom and ensure the flour is totally mixed in.
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Divide the batter evenly between the prepared cake pans and bake 30 to 40 minutes, until a cake tester inserted into the center of the cake comes out clean.
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Set the pans on a wire rack to cool for 20 minutes, then run a knife around the edge of each cake layer. Gently invert each cake layer onto wire racks to cool completely before frosting.
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(Once the cake layers are cool you can wrap them tightly with plastic wrap and store at room temperature for up to a day before frosting.)