Preheat oven to 350F and prepare three 6-inch cake pans.
In a medium-sized bowl, whisk together flour, cornstarch, baking powder, baking soda, and salt. Set aside.
In a small bowl, whisk together sour cream, milk, and vanilla extract. Set aside.
In the bowl of a mixer, beat butter and sugar at medium speed until smooth and creamy.
Add egg whites to the butter mixture, in a few increments, making sure the previous addition has been fully mixed in before adding the next.
With the mixer on low, add in half of the dry ingredients followed by the wet ingredients. Add the second half of the dry ingredients and mix until combined.
With a rubber spatula, gently fold in the chopped Oreo cookies and sprinkles.
Divide the batter between the three pans and bake for 25 to 30 minutes or until a toothpick inserted into the center of the cakes comes out clean.
Cool on a wire rack for 10 to 15 minutes before removing the cakes from their pans.
Classic Vanilla Buttercream
In the bowl of a mixer, beat butter at medium speed until smooth and creamy.
Gradually add the powdered sugar cup by cup, adding the next cup after the previous one has been mixed in.
Add vanilla extract and milk with mixer at low speed and mix until blended.
Beat at medium-high speed for 3 - 5 minutes, until buttercream is extremely smooth and fluffy.
If needed, add additional powered sugar or milk until desired consistency.