FOR THE CAKE:
Preheat oven to 350 degrees. Grease and flour 2-9 inch cake pans or line cupcake pans with liners.
In a medium bowl mix flour, baking soda, and salt.
In another bowl add buttermilk, vinegar, vanilla and eggs and whisk to combine.
Sift cocoa into a small bowl the mix with food coloring until it forms a paste.
Beat butter and sugar together, in a standing mixer, set on medium, for 2 minutes. Scrape down bowl.
Add 1/3 of the flour mixture and beat on medium speed just until it's incorporated. Add 1/2 the buttermilk mixture and beat on low until combined. Scrape down the bowl. Add 1/3 of the flour mixture and beat on medium until incorporated. Add the rest of buttermilk mixture beat on low until combined. Add last 1/3 of the flour mixture and beat on medium until just combined. Scrape down the bowl.
Add the cocoa paste mixture and beat on medium until incorporated. Scrape down the bowl and make sure the bottom of the batter is incorporating as well. Give the batter a final good stir with a rubber spoonula to mix completely and pour into prepared pans.
Bake cake/cupcakes in preheated oven about 25 minutes for cake and 20 minutes for cupcakes or until a toothpick inserted in the center comes out clean. ( I had to bake my cupcakes more like 24-25 minutes but my oven is wonky and doesn't seem to hold the temp correctly.) Cool cakes in pans 10 minutes, then turn out onto a wire rack to cool completely, at least 30 minutes. ( I skipped this step and left the cupcake in the pan and refrigerated overnight. I pulled them out the next day to frost and it worked perfectly for me.)