Prepare two round 9-inch pans with greased sides and parchment bases; set the oven to 325°F. Combine cocoa, salt, baking soda, and flour with a whisk.
Beat eggs, then mix with the batter sugar, oil, buttermilk, and 1 1/2 teaspoons vanilla. Mix with dry ingredients on low just until combined; fold in vinegar and liquid red coloring evenly.
Divide batter between the pans, tap gently, and let stand five minutes. Bake 25–30 minutes or until a tester has no wet batter and the centers spring back. Continue checking if needed.
Allow the layers to firm in their pans, then release carefully onto racks. Cool fully; allow about an hour, longer if needed.
Beat 1 cup butter with 16 ounces cream cheese. Gradually beat in 8 cups powdered sugar, then the frosting vanilla and milk. Spread between the cooled layers and over the top and sides; cut into 12 slices.