Heat oven to 350° or 325° for glass. Remove wrappers from peppermint patties. Grease 13 x 9 x 2 baking dish. Stir together butter, sugar and vanilla in large bowl. Add eggs; beat until blended. Stir in flour, cocoa, baking powder and salt. Blend well. Remove 2 cups batter, set aside.
Spread remaining batter in prepared pan. Arrange peppermint patties about 1/2 inch apart in a single layer over batter.
Spread reserved batter over patties. Bake 45 to 50 minutes or until brownies begin to pull away from the sides of dish/pan. Let brownies cool in pan on a wire rack until completely cooled. Refrigerate overnight. Remove from refrigerator and cut into squares.
Make mint cream cheese frosting:
Beat cream cheese and butter on medium speed with an electric mixer until creamy. Gradually add sugar, beating on low speed until smooth. Add peppermint extract. Beat until smooth and creamy. Frost cooled brownies with mint cream cheese frosting. Decorate with green rock candy if using.