Preheat the oven to 350°F (180℃) and line your cupcake tin with 12 liners. Set aside.
Using a hand mixer or stand mixer with the paddle attachment, beat the butter and oil together until combined. Add in the maraschino cherry juice, egg whites, and vanilla extract.
Mix in the granulated sugar.
Add in flour, cornstarch, salt, and baking powder to a separate bowl and whisk together.
Alternate slowly adding in the milk and the dry ingredients. Mix until combined.
Fold in the chopped maraschino cherries (make sure they are chopped very small), until they are evenly distributed in the batter.
Fill the cupcake liners ⅔ full, then bake for 13-17 minutes or until a toothpick inserted in the middle of a cupcake comes out clean.
Let the cupcakes cool in the pan for 10-15 minutes, then remove them. While they cool, make the buttercream.