Sift the flour, baking soda, cocoa powder, and salt together in a large bowl. Set aside.
Using a handheld or stand mixer, beat the butter on high speed until smooth and creamy, about 1 minute. Add the sugar and beat on high speed for 2 minutes until creamed together fairly well. Scrape down the sides of the bowl with a spatula as needed. Add the oil and beat on high for 2 minutes.
Add egg and vanilla extract. Beat on medium-high speed until combined. Scrape down the sides of the bowl with a spatula as needed.
Beat in the vinegar and the food coloring, until you reach your desired color. I used 2 Tablespoons.
With the mixer on low speed, add the dry ingredients alternating with the buttermilk, beginning and ending with the dry ingredients. Mix each addition just until incorporated. Your cake batter is ready!
Pour the batter into the prepared pan and bake for 30-35 minutes or until a toothpick inserted into the middle of the cake comes out clean. Remove from oven and cool completely before frosting.