Preheat oven to 350 degrees and line two 12-cup cupcake tins with liners.
Separate the 3 eggs and set aside.
In a large bowl, cream butter until light and fluffy, about 1-2 minutes. Add in vegetable oil and beat for another minute.
Gradually add in sugar, 1/2 cup at a time, beating until well combined. Add in egg yolks, one at a time, beating well after each addition.
In a separate bowl, sift together flour, cornstarch, baking powder, and salt. Stir in orange zest. Set aside.
In a small bowl, mix together milk, orange juice, vanilla extract, and sour cream.
Alternately add dry and wet ingredients into the creamed butter mixture, beginning and ending with the dry ingredients. Stir until just combined.
Beat egg whites until they form stiff peaks. Gently fold egg whites into the cake batter until completely combined.
Divide batter evenly in cupcake pans, filling each only 3/4 of the way full. Bake for 18-20 minutes, or until a cake tester comes out clean. Allow to cool.