In a bowl, mix together graham cracker crumbs, sugar, and melted butter. Press mixture into the bottom of a 9-inch springform pan. Bake for 8 minutes, then remove from oven and set aside.
In a mixing bowl, beat together cream cheese and sugar until smooth. Add eggs one at a time, beating well after each addition. Mix in cocoa powder.
Fold in mini marshmallows and chopped walnuts. Pour mixture over graham cracker crust.
Bake for 40 minutes or until center is almost set. Remove from oven and let cool for 10 minutes.
In a small saucepan, mix together heavy cream, chocolate chips, sugar and butter. Cook over low heat, stirring constantly, until sauce is smooth.
Pour hot fudge sauce over cheesecake, spreading evenly. Chill cheesecake in refrigerator for at least 2 hours.