Preheat oven to 350 F and line a 10x15" rimmed baking sheet with parchment paper.
Beat together butter, and cream cheese until smooth. Add sugar and beat until light and fluffy.
Add the egg and vanilla, beating until combined.
In a small bowl, stir together flour, salt, baking powder, and baking soda.
Add flour mixture to butter mixture and stir until just combined. Press into parchment lined baking sheet and bake for about 23-28 minutes or until the edges just start to take on a golden color.
Cool completely before frosting.
Buttercream
Beat butter until smooth.
Add shortening and mix until creamy.
Mix in powdered sugar and vanilla.
Add enough milk or cream to make it a nice spreadable consistency.
Spread over cooled sugar cookie bars and top with sprinkles if desired.
Notes
These sugar cookie bars are also wonderful with chocolate frosting. Of course you can use any frosting recipe you prefer.Frosted sugar cookie bars can be stored at room temperature for up to a week as long as the frosting you use doesn't need chilled. Freeze them for longer storage.