I topped my cookie bars with a simple chocolate frosting this time around, but I've also topped them with buttercream. Either way, they are delicious and absolutely perfect for large get-togethers.
Line a jelly roll pan (15x10x1) with aluminum foil or parchment paper and spray with cooking spray.
In the bowl of your stand mixer, cream together the butter, cream cheese, and sugar. Beat in the egg and vanilla until well incorporated.
With your mixer turned to low, add in the flour, baking powder, and baking soda, mixing until just combined.
Press the dough evenly into your prepared pan. The dough is sticky and it will look too thin, but it all works out. Just keep pressing it until you have an even layer over the entire baking sheet.
Bake for 20 minutes until edges begin to turn golden.
Cool completely before frosting.
For the frosting:
While the cookies are baking, prepare the frosting.
Add sugar, butter, and milk to a small saucepan over medium heat and bring to a boil. Boil for exactly 60 seconds and remove from heat. Stir in chocolate chips and continue stirring until the chocolate has melted.
Place the mixture in the bowl of your stand mixer and place in the refrigerator to cool.
When the mixture is mostly cool (it can still be a little warm, but not too hot or it won't whip - just return to the fridge if you find it not whipping good), place the bowl on the mixer and, using the wire whisk, whip the mixture with the powdered sugar until soft and fluffy. If needed add a bit of milk or heavy cream (up to 1/4 cup).