Place the bowl and whisk attachment for your mixer into the freezer.
Make the crust.
Preheat the oven to 325° F. Have a 9-inch pie plate ready.
Grind the vanilla wafers into fine crumbs using a food processor. (if you don't have a food processor you can crush them in a bowl using a pestle or a heavy-bottomed glass.) Add the cocoa powder to the crumbs and blend them in. With the motor running pour in the melted butter and run the food processor until the chocolaty crumbs are thoroughly saturated.
Press the mixture into the bottom and up the sides of the pie plate. Press them into a firm, even layer over the entire surface.
Place the crust in the oven and bake for 18 minutes. Because of the chocolate you won't be able to tell if the crust has browned but it should seem just set. Place the pie plate in a cooling rack and allow the crust to cool completely.
Toast the mini marshmallows.
Switch the oven to the broil setting. Line a small baking sheet with parchment paper. Spread the mini marshmallows over the baking sheet and do your best to leave space between them. (Though they will roll a bit when you pick up the pan!)
Place the pan under the broiler, set your timer for 30 seconds and STAY PUT! Literally: turn on the oven light and place your forehead against the oven door handle to watch the marshmallows: they will quickly start to brown. When they show lightly browned tops remove them from the oven immediately. Allow them to cool.
Make the filling:
Place the large marshmallows and milk in a large saucepan over medium heat. Stir the marshmallows constantly as they melt and become smooth: watch the mixture carefully to make sure it isn’t burning on the bottom. When it’s melted and smooth remove the pan from the heat.
Pour the melted mixture into a heat-proof bowl, cover it and place it in the fridge to chill for 20 minutes.
Once the marshmallow filling has cooled assemble the chilled bowl and whisk on the stand mixer. Pour the whipping cream and the cornstarch into the bowl. Beat the cream on high for about 3-4 minutes, until stiff peaks form.
Remove 1 cup of the whipped cream and set it aside.
Add the softened cream cheese to the mixing bowl and gently beat it into the whipped cream. Add to this the cooled marshmallow mixture and the Baileys Irish Cream and gently mix everything together. Sprinkle in the cooled, toasted mini marshmallows and stir them through.
Assemble the pie.
Spoon half of the filling into the completely cooled cookie crust and spread it into an even layer. Spread half of the mini chocolate chips evenly over the filling. Spoon the remaining filling over the chocolate chips and spread it evenly to the edges.
Take the 1 cup of whipped cream you set aside and whisk into it the espresso/coffee and confectioner's sugar. Spread this over the top of the pie.
Sprinkle the remaining mini chocolate chips around the outside edge of the pie, pressing them in gently. Cover the pie with plastic wrap and place it in the fridge to chill and set overnight (for at least 8 hours.)
If you're using the green decorating sugar, use a stencil or cookie cutter and a toothpick to trace a shamrock in the center of the chilled pie. Carefully spoon the decorating sugar into the shape to create the shamrock.
Slice the pie - the filling will be very soft but should stay in place when you remove the slice. Serve the slices immediately.
Plan for the overnight chill when getting ready to make this pie!
In my photos I've added a green shamrock made of sprinkles in the center. You can add this too, the next day after the pie has chilled. Or, just leave this decoration off and serve the pie the way it is.
When you slice it the filling will still be very soft, but it should stay in place if you cut and remove the pieces gently.
Cocoa powder added to the 'Nilla wafer crumbs made this a chocolate cookie crust. If you'd rather stick with a vanilla crust, just omit the cocoa.