Place plums in a large saucepan, cover and heat slowly. Gently cook plums until soft. Stir in 1/4 of the sugar and transfer everything to a large ovenproof dish.
Place in oven on the second shelf from the bottom. Allow plum jam to slowly thicken in the preheated oven with the oven door slightly ajar (stick a wooden spoon in the door). After half an hour, stir in another 1/4 of the sugar and keep baking. Repeat twice with remaining sugar.
The jam must be thick enough that if you draw a line into it with a wooden spoon, the line stays visible. Keep in oven longer if necessary.
When finished stir in Amaretto. Fill hot sterilized jars with jam and seal.