Pre-heat the oven to 350 degrees and prepare a 9x13 pan wither foil or parchment paper sprayed with non-stick spray.
In a large mixing bowl, beat together the cream cheese, butter, and sugar for about 2-3 minutes or until it's very light and fluffy. (I use a hand mixer.)
Add in the egg and vanilla extract and beat again just until combined.
Now I know I should say to get out another bowl for the dry ingredients, but no one wants to clean more bowls. So just sprinkle the all-purpose flour evenly over the batter. Then sprinkle the baking powder and salt over the flour. (I do this method all the time and it works perfectly as long as you sprinkle evenly.)
Mix the dry ingredients into the wet using a spatula or a mixer on the lowest setting. You don't want to over-mix the dough.
Add the cookie dough to the prepared pan. Press it into an even layer. I spray a little cooking spray on my clean hands and it makes It so easy to press in by hand with no sticking.
Bake in the pre-heated oven for 20-25 minutes or until you see just the edges start to turn slightly golden brown.
Remove from the oven and let cool completely before frosting.
Frosting
Using a hand-mixer, beat together the softened cream cheese, butter, and vanilla extract.
Add in the powdered sugar and beat to combine.
If you want to thin the frosting just a bit, add in the tablespoon of milk.
Frost the cooled sugar cookie bars and add sprinkles to decorate.
Since these bars have cream cheese in the icing, they should be stored covered in the fridge for no more then 4-5 days. Keeping them cool also keeps the icing nice and firm. I just bring them to room temp to serve!
Notes
-If you need to soften the butter and cream cheese before starting, you can use the microwave. Just be sure not to melt them. (I usually only need 11-12 seconds for a whole stick of butter and about 15 seconds for the cream cheese...but everyone's microwaves can differ so watch them carefully! -I use parchment or foil when I make these because I like to lift the whole thing out of the pan after frosting. You can just use cooking spray if you plan on leaving them in the pan.