Preheat oven to 375 degrees. Butter a 13" x 9" baking dish, set aside.
In a mixing bowl whisk together flour, baking powder and salt for 20 seconds, set aside.
Add sugar and lemon zest to the bowl of an electric stand mixer and with your fingertips rub lemon zest with sugar until sugar is slightly yellow and fragrant. Add butter and whip butter and sugar mixture until pale and fluffy. Stir in egg then egg white. Add sour cream, lemon extract and vanilla extract and mix until combined. Slowly add dry mixture and mix just until combined.
Spray hands with non-stick cooking spray or rub with butter and gently press mixture into an even layer prepared baking dish. Bake until toothpick inserted into the center comes out clean and bars are set, about 15 - 16 minutes. Cool completely, then frost add sprinkles and cut into squares.
For the frosting:
In the bowl of an electric stand mixer fitted with the paddle attachment, whip butter until light and fluffy. Mix in powdered sugar, lemon zest and lemon juice and mix until light an fluffy. Tint with food coloring if desired.
*You want the butter to still be cool, this is so the dough isn't too warm and sticky to work with when pressing into the baking dish.