Preheat oven to 350° F. Grease three 8-inch cake pans and line with parchment paper. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment, cream together butter, sugar and salt until smooth. Add eggs one at a time and mix until incorporated.
In a small bowl, combine cocoa powder, liquid food coloring, and vanilla extract to make a thick paste. Add paste to mixing bowl. Mix on low speed until paste is incorporated into batter.
Alternate adding flour and buttermilk to mixture. Add 1/3 of flour, followed by half of buttermilk. Start and end with flour. Mix until batter is smooth.
In a separate bowl, combine baking soda and vinegar. Add mixture to batter and mix until combined.
Transfer batter evenly among bake pans. Level out batter to ensure even layers. Bake for 20-25 minutes, rotating pans halfway thru baking. Let cake cool in pan for 10 minutes before unmolding. Allow cake layers to cool to room temperature before using.