Put the room temperature cream cheese into the bowl of a stand mixer with paddle attachment.
Mix the cream cheese for about 30 seconds to 1 minute. Slowly add the sweetened condensed milk and continue to mix on medium-high speed for about 2 minutes.
Using a spatula, scrape down the sides and then continue to mix for another minute.
Turn off mixer and add in cocoa and sugar.
Starting on low, increase speed to medium-high and mix until incorporated, about 1 minute.
Remove bowl from stand mixer and fold in whipped cream by hand.
Pour mixture into prepared graham cracker crust and place in the freezer for at least 2 hours, up to overnight. (If chilling overnight, remove cheesecake from freezer after 1 hour and secure with plastic wrap.)
When ready to serve, remove cheesecake from the freezer. Using a sharp paring knife, run the knife between edge of the springform pan and graham cracker crust. Open the clasp on the pan and press up from the bottom, allowing the external ring to fall down.
Place cheesecake on a baking sheet and cover with marshmallows, allowing them to pile up and be stacked highest in the middle.
Place cheesecake, that is covered in marshmallows in oven that has been set to BROIL.
Leave cheesecake in oven for 1-2 minutes, or until marshmallows start to brown and melt together.
Remove from oven and carefully transfer cheesecake to serving plate.
Serve immediately.
Optional: Sprinkle cheesecake with remaining graham cracker crumbs.