How to Make It
Heat oven to 350° F. Butter two 8-inch round cake pans and dust with flour, tapping out the excess. In a medium bowl, whisk together the flour, cocoa, baking soda, and salt; set aside.
Using an electric mixer, beat the butter and sugar on medium-high until fluffy, 2 to 3 minutes. Beat in the eggs, vanilla, vinegar, and food coloring until incorporated, scraping down the sides of the bowl as necessary.
Reduce mixer speed to low. Add the flour mixture in 3 additions and the buttermilk in 2 additions, beginning and ending with the flour mixture. Mix just until combined (do not overmix).
Divide the batter between the prepared pans and bake until a toothpick inserted in the center comes out clean, 40 to 45 minutes. Cool the cakes in the pans for 15 minutes, then turn out onto racks to cool completely.
Transfer one of the cooled cakes to a plate and spread with 1 to 1½ cups of the frosting. Top with the remaining cake and spread the top and sides with the remaining frosting. Refrigerate for 30 minutes before serving.