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.
Frosting
In a medium bowl, beat together the butter, 2 cups powdered sugar, and milk until combined. Slowly add in the remaining two cups of powdered sugar until combined. Add the birthday cake extract and salt.
*If you want color, add drops of desired color of food dye into the frosting and mix until fully blended and has reached desired color.
Frost with frosting and sprinkle with lots of sprinkles! Cut into bars and serve!