Sift cake flour, baking powder, and salt into a medium sized bowl.
Preheat oven to 350 degrees. Spray three 9-inch round cake pans with cooking spray OR grease and flour them.
In a large mixing bowl, cream 1 cup unsalted butter, 1/3 cup vegetable oil, and 2 1/2 cups sugar on high until fully incorporated and light and fluffy, about 5 to 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.
With the mixer on slow speed, alternate adding in flour mixture and sour cream, ending with flour until mixed through.
Do not overmix.
Divide batter evenly between prepared pans.
Bake in preheated oven for 23-30 minutes or until fully baked. (Over-baking will dry these cakes out).
11. Place on cooling rack for 10 minutes.
Remove cakes from pan and cool completely.
Use last four ingredients for Caramel Icing.
Add butter, evaporated milk, and 2 cups sugar to saucepan over medium heat until everything has melted together.
Leave over medium to low heat, stirring periodically, for about 1 1/2 to 2 hours (keeping an eye on it so it doesn't burn) until thickened and caramel has darkened to a golden brown. It should coat the back of a spoon. Reduce heat while cooking if necessary to prevent burning.