-
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.