Prepare brownie mix according to package directions, using the 13x9x2-inch pan option. Cool completely on a wire rack; set aside. Cut brownies into bars. Using about 3/4 of the bars, press brownies into the bottom of a 9x3-inch springform pan, forming an even crust. Spread with the fudge ice cream topping; set aside. Crumble remaining 1/4 of the brownies; set aside.
In a large mixing bowl, combine cream cheese and peanut butter. Beat with an electric mixer on medium speed for until just combined. Add sweetened condensed milk. Beat until just combined. In a medium bowl, beat whipping cream until soft peaks form. Add powdered sugar. Beat until stiff peaks form. Reserve 1/2 cup of the whipped cream mixture. Fold remaining whipped cream into the cream cheese mixture.
Spread one-half of the cream cheese mixture over the hot fudge topping in pan. Sprinkle with one-half of the reserved brownie crumbles. Spread remaining cream cheese mixture over the brownies. Top with the reserved 1/2 cup whipping cream. Sprinkle remaining brownie crumbles and chopped peanut butter cups. Drizzle with chocolate syrup. Cover and chill for 4 to 24 hours.