In a large, heavy saucepan over medium-low heat, bring the milk and butter to a simmer.
Reduce heat to low.
Stir in the mini marshmallows until melted.
Add the chocolate and stir in until melted and combined.
Remove from heat.
In a separate bowl, beat 1 1/2 cups of the heavy cream to form stiff peaks.
Fold the whipped cream into the chocolate mixture.
Carefully pour the chocolate whipped cream mixture into the pie crust.
Press a sheet of plastic wrap to the surface of the pie filling.
Chill for at least 3 hours, or until filling is firm. The pie will continue to set up as it chills.
Just prior to serving, whip the remaining 2 1/2 cups heavy cream with the powdered sugar until it forms stiff peaks.
Spread the whipped cream over the chilled chocolate filling.
Video
Notes
You can use any milk from skim milk to whole milk in this pie.
I used a pre-made chocolate cookie crust because I haven't been able to get chocolate wafer cookie or chocolate graham crackers. If you prefer, feel free to make a homemade crust for this pie.
The whipped cream is lightly sweetened. Feel free to add more sugar to taste.
I topped the pie with chocolate curls for looks. The pie is also delicious served without chocolate curls.