Preheat oven to 350F degrees. Line a 9x13 baking pan with parchment paper.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and sugar until fluffy. Then, beat in the eggs and vanilla.
In a separate bowl, whisk together the flour, baking powder and salt.
With the stand mixer running on low speed, slowly add the flour mixture until everything is incorporated, scraping down the sides of the bowl if necessary.
Transfer the cookie bar batter to the prepared baking pan. Bake 20 minutes, until a toothpick inserted in the center comes out clean. It will still be pale in color.
Make the frosting: In a stand mixer or with an electric hand mixer, beat the butter and cream cheese until fluffy. Slowly incorporate the powdered sugar and vanilla.
Frost the cookie layer generously. Decorate with festive sprinkles. Then, use a sharp knife to cut 15 squares.