Preheat your oven to 325°F (160°C) and prepare a springform pan lined with parchment paper. Create a water bath by wrapping the bottom of the pan with aluminum foil.

In a large mixing bowl, beat together cream cheese and granulated sugar until smooth. Add in eggs one at a time, mixing well after each addition.

Gently incorporate sour cream, cocoa powder, and chocolate cookie crumbs into the cream cheese mixture until fully combined.

Pour the cheesecake filling into the prepared crust, smoothing the top. Place in the preheated oven inside the water bath. Bake for 45-50 minutes.

After baking, turn off the oven and crack the door open, allowing cheesecake to cool for about an hour before refrigerating for at least 4 hours.

For the topping, whip heavy whipping cream until soft peaks form. Fold in marshmallow fluff until combined.

Spread marshmallow whipped cream over the chilled cheesecake and garnish with chocolate shavings or mini marshmallows, then serve.