Line the bottom of a loose-bottomed springform cake tin with non-stick paper. I used a 20cm cake tin, but use a 24cm tin for a thinner, larger cheesecake.
Combine the crumbed gingernut biscuits with the melted butter, and mix thoroughly. Press the mixture into the base of the cake tin, and place in the fridge.
Melt the butter in a saucepan over a medium-low heat, and add the chopped rhubarb, caster sugar, cinnamon, and Amaretto (if using). Cook for around 5-8 minutes, stirring occasionally, until the rhubarb is soft but still holding its shape. Drain away any excess liquid, but keep it to drizzle over the cheesecake when serving. Set aside the rhubarb to cool.
In a large bowl, combine the caster sugar, vanilla extract, cream cheese, eggs and sour cream, and whisk together until smooth.
Heat the oven to 160°C (Gas Mark 2-3 / 315°F).
When the rhubarb has cooled a little, remove the cheesecake base from the fridge, and add the rhubarb. Spread it around into an even layer.
Use a large piece of kitchen foil to create a watertight seal around the cake tin, and place it in a baking dish. Pour an inch or two of water into the dish, making sure that it doesn't get inside the foil seal.
Pour the cheesecake mixture into the cake tin on top of the rhubarb.
Bake the cheesecake in the bain marie for 1-2 hours, depending on the size of your cake tin (a smaller cake tin will produce a thicker cheesecake, and will take longer to bake). My cheesecake, in a 20cm cake tin, took around 2 hours to bake. When cooked, it can still be slightly wobbly in the centre, but should be mostly set and no longer liquid.
To brûlée the cheesecake, sprinkle over a little more sugar, and caramelise it using a chef's blowtorch, or under a moderate grill (broiler). This is best done just before serving.
Serve drizzled with the rhubarb syrup that you drained from the rhubarb earlier.