Preheat the oven to 350 degrees Fahrenheit, and line a cupcake pan with liners.
Make your "buttermilk" by combining the oat milk and apple cider vinegar in a small bowl. Stir, set aside, and let sit for 5 minutes.
In a stand mixer, add in the sugar, and oil. In another medium bowl, sift the flour, cocoa powder, baking soda and salt together.
Once the buttermilk has been left to sit for at least 5 minutes, in a small food processor or blender, add the beets, and half of the buttermilk mixture. Blend until well combined and smooth, about 10-20 seconds.
Add the beet mixture and milk to the stand mixer that has the oil and sugar. Whisk to combine well.
Using the paddle attachment for your mixer, add in half the flour. Mix until combined. Scrape the bowl down, add in the last half, and mix well again. Using a 1/3 cup scoop, fill the cupcake pan. Bake in the oven for 20 - 24 minutes, or until a toothpick inserted in the center comes out clean. Resist the urge to open the oven door, as this may cause them to sink.
When the cupcakes are done, remove them from a pan, and set on a wire rack to cool.