Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans, line them with parchment paper, then grease and flour again for good measure. This ensures easy release after baking.
In a large mixing bowl, cream together the softened butter and granulated sugar until the mixture is light and fluffy. This takes about 3-4 minutes with a mixer on medium speed.
Slowly add in the vegetable oil. Then add eggs one at a time, mixing well after each addition. You’re looking for a creamy, well-emulsified batter at this stage.
Stir in the red food coloring, white vinegar, and vanilla extract. The batter should be vibrantly red and glossy.
In a separate bowl, sift together the cake flour, cocoa powder, salt, and baking soda. Sifting helps lighten the flour and prevents any lumps in the batter.
With your mixer on low speed, alternate adding the dry ingredients and the buttermilk to the wet mixture—starting and ending with the dry. Mix until just combined. Be careful not to overmix; that’s the key to keeping the cake tender.
Scrape down the sides and bottom of the bowl to make sure everything is incorporated. Divide the batter evenly between the prepared pans. Tap each pan on the counter to remove air bubbles.