For the Brownies:
Preheat oven to 350 degrees F (175 degrees C). Grease and flour an 8-inch square pan.
In a large saucepan, melt 1/2 cup butter. Remove from heat, and stir in sugar, eggs, and 1 teaspoon vanilla. Beat in 1/3 cup cocoa, 1/2 cup flour, salt, and baking powder. Spread batter into prepared pan.
Bake in preheated oven for 25 to 30 minutes. Do not overcook.
For the Frosting:
In a stand mixer, cream together cream cheese and butter until smooth. Add the vanilla extract and mix. At med-low speed, mix in the powdered sugar. Beat on medium high about 2-5 minutes.
If the frosting is too thick, add milk one tsp at a time until you reach the consistency you want. Crush the Oreos into crumbs. With a rubber spatula or spoon, mix the Oreos into the frosting.
Allow brownies to cool for at least one hour before frosting. Keep refrigerated.