Ultimate Reese’s Cheesecake Brownies

brownie mix + ingredients on box 8 oz cream cheese, softened 2 tbsp. butter, softened 14 oz sweetened condensed milk 1 tbsp cornstarch 1 egg 1 tsp vanilla 2 c chopped peanut butter cups 1/4 c chocolate chips, melted 1/4 c peanut butter chips, melted 1 c Reese’s Pieces