Preheat your oven to 350°F (177°C) and prepare two 6-inch cake pans.
Whisk together flour, cocoa powder, baking soda, and salt in a medium bowl.
Mix butter and sugar in another bowl at medium speed, then add egg yolks, oil, vinegar, and vanilla extract and beat at high speed.
Alternate adding dry ingredients (from Step 2) and buttermilk to the mixer, then mix in ⅔ to 1 ⅓ teaspoons of gel/liquid food coloring.
Whip egg whites at high speed 'til fluffy and fold into batter.
Divide the batter between baking pans and bake for 30 to 32 minutes.
Cool cakes completely in pans on wire racks.