Position a rack in the center of the oven and preheat to 325°F (165°C). Wrap the outside of a 9-inch springform pan with two layers of heavy-duty aluminum foil, ensuring it's watertight. This will protect the cheesecake from the water bath.

In a medium bowl, combine the gingersnap cookie crumbs and granulated sugar. Pour in the melted butter and mix with a fork until all the crumbs are evenly moistened.

Press the crumb mixture firmly and evenly into the bottom and about 1 inch up the sides of the prepared springform pan. Use the bottom of a glass to pack it tightly. Bake for 10 minutes. Set aside to cool on a wire rack while you prepare the filling.

In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl with a hand mixer, beat the softened cream cheese on medium-low speed until completely smooth and creamy, about 2 minutes. Scrape down the sides and bottom of the bowl.

Add the granulated sugar and beat on medium-low speed until well combined and smooth. Add the eggs one at a time, mixing on low speed just until each egg is incorporated. Do not overmix, as this can cause cracks.

On low speed, mix in the room temperature eggnog, dark rum, vanilla extract, nutmeg, cinnamon, and all-purpose flour. Mix just until the batter is smooth and combined, scraping the bowl as needed.

Pour the filling over the cooled crust and spread it into an even layer. Place the foil-wrapped springform pan inside a large roasting pan.

Carefully pour boiling water into the roasting pan until it comes about halfway up the sides of the springform pan. Be careful not to splash water into the cheesecake.

Bake for 65-75 minutes. The cheesecake is done when the edges are set and the center 2-3 inches still have a slight jiggle (like Jell-O). It will continue to set as it cools.

Turn off the oven and crack the door open. Let the cheesecake cool in the oven for 1 hour. This slow cooling process is another key step to prevent cracking.

Carefully remove the cheesecake from the water bath and take off the foil. Let it cool completely on a wire rack at room temperature. Once cooled, cover loosely with plastic wrap and refrigerate for at least 6 hours, or preferably overnight.

Just before serving, prepare the optional whipped topping. In a chilled bowl, beat the heavy cream, powdered sugar, vanilla, and nutmeg with a mixer on high speed until stiff peaks form.

Run a thin knife around the edge of the cheesecake before releasing the springform pan's sides. Top with the spiced whipped cream, dust with extra freshly grated nutmeg, slice, and serve chilled.