Preheat oven to 325 degrees
In a medium bowl, sift together cake flour, baking powder, baking soda, and salt
Into the bowl of an electric stand mixer add butter and vegetable oil
Cream together butter and vegetable oil until completely combined (2-3 minutes). Scrape down the sides of the bowl
With the mixer on low speed, gradually pour in sugar. Turn mixer to medium to high and mix until light and fluffy 1-2 minutes
Scrape down the sides of the bowl. With the mixer on low speed, add the egg whites one at a time. Allow each to combine before adding in the next
Add in vanilla extract mix until combined. Scrape down the sides of the bowl
With the mixer on low speed, add one third of the flour mixture into the bowl and mix until just combined. Scrape down the bowl and add one half of the butter milk and again mix until just combined. Repeat this process. Begin and end with the flour mixture. Do not over mix or your cupcakes will become quite dense
Distribute cupcake batter evenly between 24 prepared cupcake liners
Bake for 18 minutes or until a toothpick inserted into the centre of the cupcakes comes out clean