Preheat the oven to 350°F/180°C and line a 12-cup cupcake pan.
In a medium bowl, whisk the flour, cornstarch, baking powder, and salt together. Set this aside for now.
Prepare the buttermilk by quickly heating it in the microwave for 10-15 seconds to bring it to room temperature, and then stir in the vanilla.
In the bowl of a stand mixer fitted with the paddle attachment, beat the softened butter, oil, and sugar together until fluffy (2-3 minutes). Then scrape down the sides of the bowl and add the egg, followed by the egg white, and continue beating on medium-high speed for another 3 minutes.
Reduce the speed and add ¼ of the flour mixture, followed by all of the buttermilk, and then the remaining flour. Mix until the flour is just combined. Then, turn off the mixer and fold ¼ cup of sprinkles in using a flexible rubber spatula.
Spoon batter into the cupcake pans, filling to no more than ¾ full. You'll get 15-16 cupcakes. Bake for 20-23 minutes, or until cupcakes are set. Let the cupcakes cool for 10 minutes in the pan before transferring them to wire racks to cool completely.
To make the frosting, start by beating the butter and cream cheese together until smooth. Then add the powdered sugar, salt, and vanilla. Starting on low speed, begin bringing the ingredients together, slowing increasing the speed up to medium-high once you're confident that powdered sugar won't go flying everywhere.
Add the heavy whipping cream, and then. continue beating at medium-high speed for 3-4 minutes until the frosting is light and fluffy, stopping to scrape down the sides of the bowl if needed.
Spread or pipe the buttercream onto the cooled cupcakes. Top with more sprinkles and enjoy!