Combine the graham cracker crumbs, sugar, peanut butter, and butter until combined. Add water, 1 tablespoon at a time, until the mixture holds together when pinched.

Press the mixture into the bottom and up the sides of the prepared pan. Bake 10 minutes or until the edges of the crust are lightly browned.

Using an electric mixer on medium speed, beat the cream cheese and sugar until fluffy. Mix in the vanilla. Add the milk in three portions, alternating with the cocoa powder, and mix until combined. Begin and end with the milk.

In a large bowl, beat the whipping cream with a whisk or with an electric mixer with the whisk attachment. Whisk until soft peaks form.

Gently fold the whipped cream into the chocolate mixture. Gently stir in the chopped peanut butter cups.

Transfer the filling to the pie crust. Spread evenly and smooth the top. Garnish with peanut butter cups.

Refrigerate at least 6 hours. Alternately, place in freezer for at least 4 hours.