Preheat the oven to 375 F and line a cupcake tin with 8-9 muffin liners (8 will be high, 9 will be standard).
Beat together the butter vanilla, salt and sugar in a bowl with a hand mixer for at least 5 minutes. The mixture should be very light and creamy. Add the egg and beat for another minute or two, until it’s well combined.
Add the baking soda, flour and cocoa on top. Then, with the mixer on medium, slowly pour in the buttermilk and squeeze in the food coloring. Beat until you have an airy smooth batter but don’t overbeat (a minute at most).
Divide the batter between cupcake liners (⅔ of the way) and bake for 20 minutes, until the tops spring back when pressed. Cool in the tin for a minute or two then remove the cupcakes and let them cool fully at room temperature. If you are making them days ahead of time, put them in an airtight container in the freezer.