Preheat oven to 350*F. Grease 2-9" round cake pans, line with parchment and dust with cocoa powder. Whisk flour, baking soda, and salt together. In another medium bowl, whisk buttermilk, eggs, vinegar, and vanilla together. In a small bowl, mix the 2 Tbsp of cocoa powder with red food coloring to make a smooth paste. In a large bowl, beat the butter and sugar together on med-high speed until light and fluffy, about 3-6 minutes. Reduce speed to low and beat in 1/3 of the flour mixture, followed by 1/2 of the buttermilk mixture. Repeat with half of the remaining flour mixture, remaining buttermilk and then the last of the flour. Mix each addition just until combined before moving on.  Beat in cocoa paste until the batter is uniform. Scrape the bowl to make sure it is all evenly incorporated. Scrape into prepared pans, smooth the tops with an angled spatula and gently tap the pans on the counter to remove any unwanted air pockets.  Bake until a toothpick comes out of the center with just a few moist crumbs attached, about 25 minutes. Rotate pans half way through baking. Let the cake cook in the pans 10 minutes and then remove to a wire rack to cool completely.