Preheat the oven to 350°F (180°C). Grease two 6-inch (15 cm) cake pans with butter and line the bottoms with parchment paper.
In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt. Set aside.
In a small liquid measuring cup, mix together the buttermilk and red gel food coloring until fully combined. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment, beat the butter and sugar on medium-high speed for 2-3 minutes until pale and fluffy. Scrape down the bowl halfway through with a silicone spatula.
With the mixer running on medium speed, slowly drizzle in the oil. Mix until well combined.
Add the egg and egg yolk one a time, mixing well in between each addition. Scrape down the bowl again and mix in the vinegar and vanilla extract.
With the mixer on low speed, add half the buttermilk followed by half of the flour mixture. Repeat with the remaining buttermilk and flour. Mix until just combined.
Scrape down the bowl once more to ensure everything is well incorporated. Do not overmix.
Transfer the batter evenly into the prepared cake pans and smooth out the tops.
Bake for 35-40 minutes or until a cake tester inserted into the center comes out clean. DO NOT open the oven until it's almost done baking.
Let the cakes cool for 5-10 minutes in the pan then invert onto a wire rack. Cool completely before frosting, about 45 minutes.