Preheat oven to 325°F. Wrap the bottom and sides of a 9-inch springform pan tightly with a double layer of heavy-duty aluminum foil to prevent water seepage.

Pulse the cookies in a food processor until fine crumbs. Pour in melted butter and pulse until mixture looks like wet sand. Press firmly into bottom and about halfway up sides of prepared pan. Bake for 10 minutes, then set aside to cool slightly.

Beat softened cream cheese and sugar on medium speed for about 2 minutes until completely smooth with no lumps. Scrape down bowl.

Add sour cream, hot chocolate mix, vanilla, salt, and flour. Beat on low speed until just combined. Add eggs one at a time, beating on low after each addition until only just incorporated.

Pour filling into slightly cooled crust. Place springform pan into a larger roasting pan. Carefully pour hot water into roasting pan until it comes about halfway up sides of springform pan.

Bake for 60-75 minutes. Edges will be set but center will have a slight jiggle (should move like Jello, not liquid).

Turn oven off, crack door open about 4 inches, and let cheesecake sit in warm oven for 1 hour for gradual cooling.

Remove from oven and water bath, then let cool completely on wire rack for about 2 hours. Cover and refrigerate for at least 6 hours, overnight is best.

Just before serving, combine cold heavy cream, marshmallow fluff, and vanilla in chilled bowl. Whip with mixer on high speed until stiff peaks form. Spread or pipe onto chilled cheesecake and garnish with chocolate shavings or mini marshmallows.