Preheat the oven to 425 degrees Fahrenheit and lightly grease your donut pan. Set aside.
In a small bowl, mix together the dairy-free milk and apple cider vinegar. Then set aside for 5-10 minutes as it begins to activate.
In a mixing bowl, whisk together the flour, pure cane sugar, cocoa powder, baking powder, sea salt, and xanthan gum (if needed) and make a small "well" in the dry ingredients for the wet ingredients to be poured into.
Next, add the milk/apple cider vinegar mixture along with the egg (or egg replacer), vegetable oil, and food coloring to the well and mix until just combined. (Add more or less food coloring to get the desired color. The more neon blue coloring added, the darker it'll be.)
Now, pour the batter into a pipping bag (OR zip loc if you have that instead), seal it, and then cut one of the corners off (make sure it's not too big, so that you can control how much comes out at once!). Fill each donut cavity at the half-way point and evenly spread batter around if needed, using a butter knife.
Bake in the oven for 8-10 minutes, or until the donut springs back when you lightly touch it.
Remove from the oven and let donuts cool slightly before removing them from the pan and onto a cooling rack.