Make the buttermilk: Pour 1 tbsp 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.