Preheat the oven to 350° F. Line a 9x13 pan with parchment paper or spray with nonstick spray and set aside.
Cream together the butter and granulated sugar, mixing until completely combined.
Next, add the eggs and vanilla. Mix until well combined.
Lastly, add the flour, cocoa powder, powdered sugar, corn starch, baking powder and salt. Mix until a soft dough forms.
Press the finished dough into the prepared 9x13 pan.
Bake at 350° F for 18-23 minutes. When it is done, the entire pan will rise slightly, and the top should no longer look wet, but will be shiny and almost ‘flakey’. The center of the bars should pass the toothpick test when they are ready to come out of the oven.
Allow the cookie bars to cool completely before frosting.
To make the frosting, start by whipping the butter and cream cheese until smooth.
Add the powdered sugar, one cup at a time, mixing until it is combined before adding another cup.
Add in the cocoa powder and the vanilla.
Finally, add the milk 1 tsp at a time until you've reached your desired consistency.
Frost the cooled cookie bars and top with sprinkles. Chill prior to cutting for perfect cuts. Enjoy!