Preheat oven to 350 degrees. Butter pans; line with parchment paper. Butter lining; dust with cocoa, tapping out excess. Set aside. Whisk together flour, salt, and cocoa in a medium bowl; set aside.
Mix sugar and oil on medium speed in the bowl of an electric mixer fitted with the whisk until combined. Add eggs one at a time; mix well after each addition. Mix in food coloring and vanilla. Add flour mixture in 3 batches, alternating with the buttermilk and beginning and ending with flour, mixing well after each addition. Scrape down sides of bowl as needed.
Stir together baking soda and vinegar in a small bowl. Add baking-soda mixture to batter, and mix on medium speed 10 seconds. Pour batter into prepared pans. Bake until a cake tester inserted into centers comes out clean, 30 to 35 minutes. Let cool completely in pans on wire racks.
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