Preheat the oven to 350F and line a muffin tray with 10 cupcake liners.
Add the oat milk and apple cider vinegar to a bowl and let sit about 5 minutes to curdle and create a vegan buttermilk. Then, add in the sugar, oil, and vanilla bean paste and whisk together.
Next add in the dry ingredients starting with the flour, baking powder, baking soda, and salt and whisk together into a batter until no streaks of flour remain. You can also sift the dry ingredients together in a separate bowl before adding if you prefer. Finally, fold in the rainbow sprinkles using a spatula.
Fill the cupcake liners with the batter so each is about ⅔ full. Then, gently tap the muffin tin on the counter to release any large air bubbles. Add to the oven and bake for 20-22 minutes until cooked through.
Remove the cupcakes from the oven, place them onto a wire rack, and let them cool completely before frosting.
Once cooled, make the frosting by adding the room temperature butter to a large bowl and whipping with an electric beater until smooth and fluffy. Then, add in the vanilla and whip another 30 seconds. Gradually add in the sifted powdered sugar and continue beating until a fluffy and smooth frosting forms. I prefer 1 ½ cups powdered sugar, but feel free to add more or less depending on preference.
Next, pipe the frosting onto the cupcakes and top with additional rainbow sprinkles. Serve and enjoy!