1. Prepare a 9-inch springform pan by lining the sides with parchment paper that sticks about an inch above the sides of the pan.
2. To make the mousse, beat the cream cheese and sugar for together in a large mixer bowl until smooth.
3. Add the milk and beat until smooth. Set cream cheese mixture aside and make whipped cream.
4. Whip heavy whipping cream on high with a mixer until it begins to thicken. Add vanilla extract and powdered sugar and continue to whip on high until stiff peaks form.
5. Carefully fold 1/3 of the whipped cream into the cream cheese mixture until combined. Add the remaining whipped cream and fold mixtures together until well combined.
6. Divide the mousse into two bowls. You’ll want twice the amount in one bowl as the other. You should have about 2 1/3 cups in one bowl and the rest in another.
7. Add the peanut butter to the bowl with the larger amount of mousse and fold to combine.
8. Add the cocoa to the smaller amount and fold to combine.
9. Pour the milk for the cookie layers into a small bowl. Place a single layer of cookies into the bottom of the pan, dipping each into the milk before adding.
10. Top the cookies with half of the peanut butter mousse and spread into an even layer.
11. Add another layer of cookies on top of the peanut butter mixture, dipping each into the milk before adding.
12. Top the cookies with the chocolate mousse and spread into an even layer.
13. Add another layer of cookies on top of the chocolate mixture, dipping each into the milk before adding.
14. Top the cookies with the remaining peanut butter mousse and spread into an even layer.
15. Refrigerate cake until firm, 4-5 hours. Once firm, remove icebox cake from pan.
16. To make whipped cream for topping, whip heavy whipping cream on high until it begins to thicken. Add the powdered sugar and cocoa powder and continue to whip on high until stiff peaks form.
17. Pipe the whipped cream around the top edge of the cake, then drizzle with chocolate sauce. Finish cake with chopped mini Reeses, if desired.
18. Refrigerate until ready to serve.