Preheat oven to 350°F and line a cupcake tin with 12 liners.
Mix the oat milk and lemon juice together and let it sit for at least 5 minutes.
In a medium mixing bowl, whisk together the flour, cocoa powder, baking soda, baking powder, and salt. Make sure all the clumps are removed, and then set it to the side.
In another medium bowl, whisk together the oat milk mixture, eggs, granulated sugar, coconut sugar, coconut oil, and vanilla until combined.
To guarantee the smoothest batter, I recommend sifting the dry ingredients into the wet ones. Add the dry ingredients to the wet ingredients and whisk just until JUST combined. Do not overmix.
Fill each cup with batter until about ½ to ⅔ full. Do not overfill or the cupcakes will spill over the edges.
Bake 18 - 22 minutes, or until a toothpick comes out clean. I baked mine for 20 minutes.
Make Frosting: Meanwhile, make 1 batch of dairy-free chocolate frosting. Let cupcakes cool for 5 minutes in the pan then transfer to a rack to cool completely to room temperature before frosting. Top with chocolate or colored sprinkles, if desired, and store in leftovers in the fridge for up to 5 days.