Preheat oven to 350 degrees or temperature on back of brownie box. Lightly grease a 9 or 10-inch spring form pan and set aside.
In large bowl, mix brownie batter according to package directions. Pour into prepared pan and bake 30-40 minutes or until set. Look on the back of the box for the time it takes to bake a 9×9 square pan of brownies-that’s a good time to start off with and then go from there. Remove brownies from oven and cool completely.
In a stand mixer, whip heavy cream with vanilla and cornstarch until firm peaks form. Scrape whipped cream into smaller bowl and set aside. In the same stand mixer bowl (no need to wash!), whip cream cheese and peanut butter together until combined. Stir in powdered sugar and vanilla. Scrape sides and whip again for 1 minute. Carefully fold in whipped cream. Scoop peanut butter mixture onto cooked brownie and smooth the top. Refrigerate until set, at least 2 hours.
When you are ready to serve, remove sides of spring form pan. Cut into pieces and drizzle with chocolate sauce and salted peanuts.