Preheat oven to 350ºF. Grease and flour 2 8x3-inch round cake pans; set aside.
In a large bowl, sift flour, cocoa powder, baking soda, and salt. Whisk well and set aside.
In a measuring cup, add buttermilk, vinegar, and gel coloring; stir to combine and set aside.
In the bowl of a stand mixer with the paddle attachment, cream butter and sugar together on medium speed until light and fluffy, about 3 minutes, scraping the sides and the bottom of the bowl.
Add eggs, one at a time, mixing well between each addition.
While the machine is running, add vanilla extract and process for 30 seconds. Gradually pour in grapeseed oil and process until blended.
Add half dry ingredients and half buttermilk mixture. Process on low speed until just combined before adding the remaining dry and wet ingredients. Process until smooth and just combined; clean the sides and the bottom of the bowl.
Divide the batter evenly between the prepared cake pans and gently tap them on the counter to dislodge any trapped air bubbles.
Transfer to the preheated oven and bake for 30 to 35 minutes or until a cake tester inserted in the center comes out clean, rotating halfway through cooking.
Remove from the heat and run a knife around the edges to loosen the sides. Let them sit in the pans for 15 minutes before inverting them onto a wire rack to cool completely.
Chill the cakes in the fridge for 5 hours or overnight.