Preheat oven to 350 degrees F. Line a 9×13 inch baking pan with parchment paper. Set aside.
Whisk the flour, baking powder, baking soda and salt together in a medium bowl. Set aside.
In a large bowl, or stand mixer, cream together the butter and sugar until light and fluffy; about 2 minutes. Add the eggs and combine well. Add the sour cream and birthday cake (or vanilla) extract; beat until combined. Gradually blend in the flour mixture just until combined.
Press dough evenly into the bottom of the greased pan with your hands. You can spray a little cooking spray on your hands so the batter doesn’t stick to them. Bake for 18 to 19 minutes only. Remove from oven and let cool completely before frosting.