Preheat oven to 350 degrees. Grease two 9-inch round cake pans with shortening (or PAM spray) and line bottom with parchment or put liners in a cupcake tin.
In the bowl of a stand mixer, combine butter and sugar until fluffy. Add the eggs and vanilla. Mix well.
In a small condiment cup, stir together liquid food coloring and cocoa powder until smooth and then add to the butter mixture.
Sift together flour, baking powder, and salt. Add flour to butter mixture in three additions, alternating with the cup of buttermilk. Begin and end with flour. Scrape down bowl and mix well.
In a small condiment cup, combine baking soda and vinegar. Mixture will bubble and fizz. Pour into the cake batter and mix until well incorporated.
Put batter in cake tin or cake pans and bake for 25-30 minutes
Let cool completely before icing.