Make the buttermilk: Pour 1 tablespoon of white vinegar into a liquid measuring cup or a regular 1 cup measuring cup. Add in the soy milk until it fills to the 1 cup mark (you should have 1 cup of liquid total). Let sit for 5-10 minutes until the milk has curdled.
Preheat oven to 350°F and line a muffin tin with 12 liners. Line a second muffin tin with 4 more liners – this recipe makes 16 cupcakes.
In a medium bowl, combine flour, baking powder, and salt and set aside.
In the bowl of a standing or handheld mixer, beat together the butter and sugar until light and fluffy, around 1-2 minutes. Add in the egg and egg whites, then add in the vanilla extract and buttermilk and mix, scraping down the sides of the bowl.
Add in the dry ingredients and mix until combined.
Fill the cupcake tins ⅔ full with cupcake batter, then bake for 12-15 minutes, or until a toothpick inserted in the centre of a cupcake comes out clean.
While the cupcakes cool, make the buttercream.
Vanilla Buttercream
Add butter and shortening to the bowl of a standing or handheld mixer. Beat together until smooth, scraping along the sides of the bowl as needed.
Mix in the vanilla extract and salt, then add powdered sugar, ½ cup at a time until desired consistency is reached. I recommend chilling the frosting in the fridge before piping it onto the cupcakes*
Once the cupcakes have cooled completely, frost the cupcakes and add sprinkles if desired!
Notes
PLEASE NOTE: I highly recommend using a kitchen scale to weigh your dry ingredients. Metric measurements are found above under the "Metric" tab.DAIRY FREE BUTTERMILK: I have tested buttermilk using almond, soy, and oat milk and I find that soy milk curdles the most (which is what you want to happen when making buttermilk), but the difference in the final cupcakes is very minor, so you can use any plant based milk you have!VANILLA EXTRACT: I like to use pure vanilla extract in this recipe has the vanilla flavor is more pronounced, but you can definitely use artificial as well!CHILLING THE BUTTERCREAM: If the frosting seems a bit soft, cover the bowl with plastic wrap and refrigerate for 15 minutes (or more). It really helps to first up the buttercream and makes it easier to pipe onto the cupcakes.Storage and Freezing Instructions: Frosted cupcakes can be stored in an airtight container or cupcake carrier at room temperature for around 2 days. If you live in a very warm climate, the buttercream should be kept in the fridge until ready to eat.Unfrosted cupcakes can be frozen for 2-3 months in a Ziploc freezer bag or airtight container. To thaw, leave them at room temperature for a few hours.Buttercream can be frozen for up to 2 months in an airtight container. Before using, let it thaw in the fridge for a couple of hours, then stir a little bit to fluff it up.