Preheat your oven to 160°C (325°F). Prepare a 23cm (9-inch) springform pan by greasing it lightly. Wrap the bottom and sides of the springform pan securely with heavy-duty aluminum foil. This water bath will help the cheesecake cook evenly and prevent cracking.

In a medium bowl, combine the crushed gingersnap cookies, 75g (1/3 cup + 1 tbsp) of granulated sugar, and 115g (1/2 cup) melted unsalted butter. Stir until the crumbs are evenly moistened and resemble wet sand. Press this mixture firmly and evenly into the bottom and about 2.5cm (1 inch) up the sides of the prepared springform pan. Use the bottom of a glass or measuring cup to ensure a compact layer.

Bake the crust for 10 minutes at 160°C (325°F). Remove from the oven and let it cool completely on a wire rack while you prepare the filling. This pre-baking step helps to set the crust and prevent it from becoming soggy.

In a large bowl, beat the softened 900g (2 lb) cream cheese with an electric mixer on medium speed until completely smooth and creamy, with no lumps. This may take 3-5 minutes. Scrape down the sides and bottom of the bowl frequently to ensure even mixing. Avoid overbeating at this stage, as too much air incorporated now can lead to cracking later.

Gradually add the 200g (1 cup) of granulated sugar to the cream cheese, beating on low speed until just combined and smooth. Scrape down the bowl again.

Add the 4 large eggs and 2 egg yolks, one at a time, beating on low speed after each addition until just incorporated. Do not overmix. Mix only until the yolk disappears. Overmixing at this stage can incorporate too much air, leading to a cracked cheesecake.

In a small bowl, whisk together the 240ml (1 cup) heavy cream, 60ml (1/4 cup) dark rum or bourbon, 15ml (1 tbsp) vanilla extract, 1/2 tsp ground nutmeg, 1/4 tsp ground cinnamon, and 1/4 tsp salt. Pour this mixture into the cream cheese mixture and mix on low speed until just combined and smooth. Scrape down the sides and bottom of the bowl to ensure everything is incorporated.

Pour the cheesecake filling evenly over the cooled gingersnap crust in the prepared springform pan. Gently tap the pan on the counter a few times to release any large air bubbles.

Place the foil-wrapped springform pan into a larger roasting pan. Carefully pour hot water into the roasting pan, ensuring the water level comes about halfway up the sides of the springform pan. This water bath (bain-marie) creates a humid environment, crucial for even baking and preventing cracks.

Carefully transfer the roasting pan with the cheesecake and water bath into the preheated oven. Bake for 60-75 minutes. The cheesecake is done when the edges are set, but the center still has a slight wobble (like Jell-O) when gently shaken. Do not overbake, as it will continue to cook as it cools.

Turn off the oven. Carefully crack open the oven door about 5cm (2 inches) and leave the cheesecake inside to cool slowly for 1 hour. This gradual cooling prevents drastic temperature changes that can cause cracking.

After 1 hour, remove the cheesecake from the oven and carefully lift the springform pan out of the water bath. Remove the aluminum foil. Let the cheesecake cool completely on a wire rack at room temperature. Once cooled, cover the cheesecake loosely with plastic wrap and refrigerate for at least 6 hours, or preferably overnight, to allow it to set fully.

Before serving, gently loosen the sides of the springform pan and remove the ring. In a small bowl, whisk together the 120g (1/2 cup) sour cream and 30g (1/4 cup) powdered sugar until smooth (optional). Spread this mixture evenly over the top of the chilled cheesecake. Alternatively, you can serve it plain or with a dollop of whipped cream.

Garnish the cheesecake with freshly grated nutmeg, if desired. Slice the cheesecake using a sharp knife dipped in hot water and wiped dry between each cut for clean slices. Serve chilled.