Preheat the oven to 350 degrees F and line a muffin tin with cupcake liners.
Combine the milk with vinegar and allow to sit for 5 minutes to allow the milk to curdle.
Whisk together the flour, sugar, baking powder, and salt in a large mixing bowl. This is to remove any clumps.
Pour the oil, vanilla, and eggs into the milk mixture, and then add to the dry ingredients in the large bowl. Mix until combined.
Fill the muffin liners 3/4 of the way full and then bake for 20-22 minutes or until a toothpick inserted comes out clean.
Carefully remove the cupcakes from the oven and allow to cook 2-3 minutes before handling. Transfer the cupcakes to a cooling rack to continue to cool.
Buttercream Recipe
Beat the butter until nice and fluffy, and add in the powdered sugar, vanilla, and milk. Add more milk if the buttercream is too thick or more powdered sugar if the buttercream is too thin. If you're not using salted butter (most vegan butter is salted), you can add 1/4 teaspoon of salt.
Add about 1 1/2 cups of the buttercream to a bowl and add in a drop of pink or blue food dye, mix until combined. Add to a piping bag and set aside.
Scoop a hole in the cupcakes and then pipe in the colored buttercream.
Top with buttercream using an icing spatula (carefully not to mix the icing) or use a piping bag with your desired tip. Decorate with your favorite sprinkles.
Notes
To make this a dairy free birthday cake we used Country Crock plant butter sticks for the buttercream and full fat oat milk for the "milk".