Preheat oven to 350 F. Place a piece of parchment paper into the base of a 9x9-inch square baking pan.
In a stand mixer, cream the butter and cream cheese with the white sugar. Once creamed, mix in the eggs, vanilla and milk and stir into very smooth batter forms - about 2 minutes.
In a mixing bowl, whisk together the flour, baking powder and salt. Pour half into the stand mixer and mix into the wet ingredient. Pour the remaining flour mixture into the cookie dough and mix until dough forms.
Spoon the dough/batter into the prepared baking dish. Spread evenly into the dish, using a spreader tool or a spatula with a bit of water on it. The water will help keep the dough from sticking and not spreading.
Bake in the preheated oven for 22 to 24 minutes, or until toothpick comes out clean. Let cool COMPLETELY before frosting.