Make the brownie layer: in a large bowl, combine the brownie mix, eggs, vegetable oil, and water. Stir until the batter is smooth and thick.
3. Step
Pour the batter into an 8-inch (20-cm) springform pan and smooth the top with a spatula or knife.
4. Step
Bake for 25 minutes. Remove from oven and use a spatula to press down the top. Let cool to room temperature.
5. Step
Make the peanut butter mousse: in a small bowl combine the gelatin and water, and set aside.
6. Step
In a large bowl, mix the peanut butter and powdered sugar until smooth and shiny.
7. Step
Microwave the gelatin mix for 10 seconds, then mix into the peanut butter until thick and lumpy. Gently fold in half the whipped cream. Once incorporated, fold in the remaining whipped cream.
8. Step
Pour into the springform pan over the brownie base and smooth the top with a spatula or knife. Freeze for 20 minutes.
9. Step
Make the chocolate mousse: in a small bowl, combine the gelatin and water, and set aside.
10. Step
In a large bowl, combine the chocolate chips and heavy cream. Microwave in 20-second increments, stirring between each, until melted.
11. Step
Microwave the gelatin for 10 seconds, then add to chocolate mixture. Gently fold in half of the whipped cream. Once incorporated, fold in the remaining whipped cream.
12. Step
Pour the batter into the springform pan over the peanut butter layer and smooth with a spatula or knife. Freeze overnight.
13. Step
Remove cake from the freezer and release the springform. Cut the chocolate peanut butter cups into sixths and evenly place around the rim of the cake, or decorate as desired!
