Preheat oven to 350 degrees and grease a muffin tin.
In a small bowl, stir the milk and apple cider vinegar together to make dairy-free buttermilk and set aside to rest for 5 minutes.
In a medium bowl, whisk together the oat flour, cocoa powder, tapioca starch, and baking soda.
In a large bowl, whisk together the apple sauce, coconut sugar, "buttermilk", and vanilla.
Add the dry ingredients to the wet ingredients and whisk together until fully combined.
Let the batter rest for 20 minutes to allow the oat flour to soak up some of the liquid.
Give the batter another quick stir and pour it into the muffin tin.
Bake for 25-28 minutes until the tops of the cupcakes feel firm to the touch.
Let the cupcakes cool for 15 minutes in the pan before placing them on a cooling rack to completely finish cooling.
Scoop the frosting into a piping bag with the tip of your choice and frost the cupcakes.