Cream sugar and butter in the bowl of a stand mixer.
Add sour cream and beat until smooth.
Add egg and vanilla and blend until smooth.
Pour the flour into the bowl and place the baking soda, powder and salt on top of the flour.
Mix in all of the dry ingredients just until the flour disappears. Do not over mix.
Grease a (11x18) sheet pan with cooking spray.
Spread the dough into the pan with a long knife or a small rolling pin.
Bake at 350 for 20-25 minutes. Cook just until golden brown.
Let cool completely.
Cream the butter and sugar in a large mixing bowl.
Add the vanilla and salt. Add milk a little at a time, until the frosting is spreading consistency.
Spread frosting on cookie bars when completely cooled. Decorate with sprinkles right after frosting.
Notes
-Sprinkling a little flour on the dough makes it easier to spread or roll out. If you are using a spreader spray it will a bit of cooking spray before spreading the dough. -Do not overtake the dough, check after 20 minutes with a toothpick, if a toothpick inserted in the center comes out clean, the cookies are done.