Preheat oven to 375.
Line a 9 x 13 pan with parchment & spray lightly with cooking spray – set aside.
In a mixer bowl with the paddle attachment, cream together butter & sugar until light & fluffy.
Mix in egg, egg white, sour cream & vanilla until well combined.
Slowly stir in flour, baking powder, & salt to form a crumbly dough.
Transfer dough to prepared pan & press into an even layer. Using a fork, poke the dough every few inches so it will bake evenly.
Bake17-20 minutes or until lightly browned around the edges.
Cool completely before frosting