Preheat your oven to 350°F (175°C) and grease and line three 6-inch cake pans. In a medium bowl, sift together the flour, cocoa powder, salt, and baking soda.

In a stand mixer bowl, beat the butter on medium speed until soft. Add the sugar and mix until light and fluffy. Add the eggs one at a time, mixing well after each.

Add the vanilla extract and beat on low speed just until combined, scraping down the sides of the bowl.

Stir the vinegar and red food coloring into the buttermilk.

With the mixer on low, add half of the flour mixture, then half of the buttermilk mixture, and mix until just combined. Repeat with the remaining flour and buttermilk, scraping down the bowl occasionally.

Divide the batter evenly between the pans and bake for about 35–40 minutes, until the centers spring back lightly or a toothpick comes out clean. Cool in the pans for 10 minutes, then turn out onto a wire rack to cool completely.

Beat the butter, cream cheese, vanilla, and a pinch of salt until smooth and fluffy. Gradually add the sifted powdered sugar and beat until creamy.

Set aside about one cup of frosting. Place one layer on a stand and spread about ½ cup of frosting on top; repeat with the remaining layers, then cover the outside of the cake with the rest.

Pipe the reserved frosting as a border or decoration, then chill the cake for about 1 hour until the frosting is firm.