Preheat oven to 375 degrees F. Spray a 9x13 pan (I use a glass Pyrex dish) with cooking spray and set aside.
In a medium bowl, whisk together flour, baking powder, and salt. Set aside.
In a small bowl, combine granulated sugar and lemon zest. Rub together with your fingers until fragrant.
In the bowl of a stand mixer, beat butter and and lemon sugar mixture together until combined. Slowly add the powdered sugar and beat until light and fluffy, about 3 minutes. Add in the eggs, lemon juice, and vanilla. Beat until smooth. Slowly add the flour mixture and beat until just combined.
Evenly spread the cookie dough into the prepared pan with a spatula. Bake for 12-14 minutes, or until the bars are slightly set in the middle. You don't want the bars to brown. Remove from the oven and let the bars cool completely.
While the bars are cooling, make the lemon cream cheese frosting. In the bowl of a stand mixer, beat the butter and cream cheese until smooth. Add the lemon zest and lemon juice. Slowly add the powdered sugar, 1 cup at a time. Mix until smooth.
Frost the cooled cookie bars with lemon cream cheese frosting. I always have a little frosting leftover, but use as much as you like! Decorate with colored sugar or sprinkles, if desired. Cut the bars into squares and serve.
Note-store bars in an air tight container in the fridge for up to 2 days.