Preheat oven to 350 degrees.
Grease 13 x 9 x 2" Baking Pan or line with greased parchment or foil sling. (2 layers of greased foil or parchment that overhang the long sides of the baking pan, helps with removal of cookie bars when cool.)
In medium bowl, mix together flour, baking powder and salt.
(I use my stand mixer with flex edge beater to mix these up, you could also use a hand mixer with (regular) turbo beaters to mix them as well.)
In large bowl, cream together butter, cream cheese and sugar. This will take several minutes - it should be fluffy and lighter in color. Beat in egg and vanilla.
Mix the flour mixture into the butter mixture until just combined. Press the dough into the prepared baking pan with greased spatula or damp hands.
Bake the cookie bars for about 20 minutes or until cookie bars are just golden on the edges. Time may vary depending on oven.
Let cool completely in pan before frosting.
In large bowl, beat together butter, vanilla, 2 cups of powdered sugar and a tablespoon milk until smooth and creamy.
Gradually add the rest of the sugar and as much milk as needed to reach desired consistency. Use food coloring to color the frosting if desired.
Frost the cookie bars and add sprinkles if desired.
Then you can store for up to 3 days in an airtight container or freeze.
If you use a darker colored baking pan you may want to drop your oven temperature by 25 degrees. Darker baking pans get hotter.
Ovens are often not accurate in their temperature...
I always keep an inexpensive oven thermometer in my oven to make sure it reaches the correct temperature.
Nutrition (approx. data estimated via online nutritional calculator.)