In a large bowl, whisk together flour, baking powder and salt. Set aside.
In another bowl, beat butter on medium low until creamy.
Add sugar and oil and beat until blended and creamy.
Add the milk, sour cream and vanilla and mix to incorporate.
Add about half of the prepared dry ingredients and stir until just combined, then add the remainder and stir until incorporated.
Scrape down sides of bowl and set aside.
In another bowl, use a hand mixer with whisk to beat egg whites until stiff peaks form.
Use spatula to gently fold whipped egg whites into batter.
Preheat oven to 350.
Grease and flour the sides of 2 8" cake pans. Cut a circle of parchment and place in the bottom of each.
Evenly divide batter into prepared pans.
Bake for 35-40 minutes until a toothpick inserted in the center of each cake layer comes out clean. Rotate pans halfway through baking.
Allow cake to cool for 10-15 minutes, before inverting on to cooling rack to cool completely.
Frost cooled cake with a double batch of buttercream frosting.