Preheat oven to 350 degrees F. Line two 9-inch round cake pans with parchment paper. Spray with nonstick cooking spray WITH FLOUR or grease and flour pans.
Whisk flour, baking soda, and salt together in a bowl. Whisk buttermilk, eggs, vinegar and vanilla together in a 2-cup liquid measuring cup. Mix cocoa with food coloring in small bowl until a smooth paste forms.
Using stand mixer fitted with the paddle attachment, beat butter and sugar on medium-high speed until pale and fluffy, 3-4 minutes. Reduce speed to low and add flour mixture in thirds alternating with buttermilk mixture, scraping down the bowl as needed. Add cocoa mixture, increase speed to medium, and beat just until incorporated, about 30 seconds (don't overmix). Give batter a final stir by hand.
Divide batter evenly between prepared pans and smooth tops with rubber spatula. Bake until toothpick inserted in the center comes out clean, about 25 minutes, rotating pans halfway through baking. Let cakes cool in pans on wire rack for 10 minutes. Remove cakes from pans, discarding parchment, and let cool completely on rack, about 2 hours.
Using a serrated knife, level the top of each cake. Use your fingers to crumble the tops into small crumbs; set crumbs aside in an airtight container to garnish the cake. Set 1 cake round in a parchment paper lined springform pan cut side up; set aside second cake round.