Add red food color, unsweetened cocoa, vinegar, vanilla extract, salt, and baking soda. Beat on high speed until well incorporated, scraping down the sides of the bowl if necessary.

Preheat the oven to 350°F (180°C). Line the bottom of an 8-inch round cake pan with parchment paper and grease the sides. Set aside. Sift together the flour, cocoa powder, and salt, then set ...