Prepare enough sterilised jars to hold 680g jam
Roughly cut the cherries and put into a medium-sized heavy-based pan with the sugar and lemon juice
Cook over a moderate heat, stirring frequently with a wooden spoon until the sugar has dissolved and the fruit has softened (10-15 minutes)
Optional step: briefly pulse the fruit with a stick blender a couple of times to break the fruit up slightly. Take care not to over process the fruit - the aim is to leave texture and lumps of fruit in the jam rather than reducing it to a puree
Add the Amaretto and the vanilla. Stir to combine
Return the pan to the hob and turn the heat up to medium-high. Allow the jam to bubble and boil, testing the temperature every few minutes with a food thermometer (I used my Thermapen) until the setting point of 105C/ 221F is reached. This will take around 10 minutes Take the pan off the heat, stir in the butter (if using) and allow to cool for 10 minutes before decanting into sterilised jars. Top with a disc of wax paper (optional)
Seal the jars immediately, turn them upside down and let cool completely before labelling
Store in a cool place for up to 12 months. Once opened, refrigerate and use within 3 weeks