Preheat oven to 375 degrees F. Line a 9x13" baking pan with parchment paper and spray with cooking spray. Set aside.
First combine dry ingredients (flour, baking powder, and salt). Set aside.
In the bowl of a stand mixer with paddle attachment (or in a large bowl with a handheld mixer), beat together butter and sugar until light and fluffy. Beat in egg and egg white, and vanilla until well combined.
Slowly add dry mixture and mix until well combined. Turn dough out into baking pan and press mixture into pan. Bake 14-15 minutes until edges have the lightest brown color. Then remove from oven. Cool completely, then frost.