Preheat oven to 375 degrees. Line an 8 inch baking pan with foil, and spray with non-stick spray.

In a large bowl combine melted butter, and cocoa powder. Mix until well combined. Add eggs and sugar, mix well. Stir in the flour and salt until well combined. Fold in peanut butter cups. Pour into prepared pan. Bake for 25 minutes, or until a tester comes out clean.

Remove from oven, and cool completely.

Meanwhile, prepare the frosting. Use a food processor to finely crush the peanut butter cups. In an electric mixer beat the butter until fluffy. Mix in cocoa powder until well combined. Add the vanilla. Mix in the sugar, it will become very thick. Slowly add the milk until you get the desired consistency. Fold in the peanut butter cups.

Once brownies have completely cooled, frost.