Preheat oven to 350 degrees.
In a large mixing bowl, cream butter, oil and sugar on high until fully incorporated and light and fluffy, about 5-6 minutes.
Turn mixer to medium speed and mix in eggs and egg yolks one at a time until well incorporated.
Add in vanilla extract and mix.
Sift cake flour, baking powder and salt into a medium sized bowl.
With mixer on slow speed, alternate adding in flour mixture and sour cream ending with flour mixture until mixed through.
Do not over mix.
Spray three 9 inch round cake pans with baking spray or grease and flour them.
Pour batter into individual cake pans evenly.
Bake in preheated oven for 23-30 minutes or until fully baked (but DON'T OVERBAKE THESE CAKES- CHECK THEM EARILER SO THEY DON'T DRY OUT).
Remove cake pans from oven and cool on cooling racks for 10 minutes.
Remove cakes from pans and wait until completely cooled to ice.