Wash plums and remove stones. Place in a large pot, cover and heat slowly. Cook plums until soft. Stir in 1/4 of the sugar and put everything in a large ovenproof dish (preferably a large roasting tin).
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 a wooden spoon in the door). After half an hour, stir in 100g sugar and keep baking in the oven. 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 plum jam into hot sterilized jars and seal.