Preheat oven to 350°F. Grease two 9"x2" round cake pans.

Sift flour, cocoa, and salt into a bowl.

In stand mixer with paddle attachment, beat butter at low speed until creamy

Add sugar; blend on medium speed, scraping bowl occasionally, untill fluffy, about 3 mins.

Add eggs 1 at a time, mixing well after each. Scrape bowl; add food coloring and vanilla.

On low speed, beat in flour mixture in thirds, alternating with buttermilk, beginning and ending with flour mixture.

In small bowl, stir together vinegar and baking soda.

Stir into batter

Divide batter between prepared pans; spread evenly.

Bake until a toothpick inserted in center comes out clean. 25 to 30 mins.