Combine the graham cracker crumbs, brown sugar, salt and butter and press into a 9-inch pie dish. Put in the freezer while you prepare the filling.
Whisk together the sweetened condensed milk and marshmallow cream until smooth. With an electric mixer, beat the heavy whipping cream to medium peaks. Gently fold into the marshmallow mixture.
Spoon the filling into the pie dish. Depending on how deep your dish is, you may not need all of the filling. Freeze for at least 2 hours before adding topping.
Spread the fudge sauce over the top of the pie and freeze until pie is completely firm, about 4-6 hours more or overnight.
When ready to serve, top with mini marshmallows and graham cracker pieces. For easier slicing, dip a sharp knife in hot water and wipe dry with a towel in between cuts.