Put 2 or 3 small plates in the freezer for testing the set later. Wash your jars and lids, then sterilise them: place jars in a 140°C (275°F) oven for 15 minutes, or boil them for 10 minutes.

Put the whole damsons and water in a large, wide, heavy-based pan. Bring to a simmer, cover and cook gently for 15 to 20 minutes, stirring now and then, until the fruit is completely soft and the skins have split.

Take the pan off the heat. As you stir, the stones will separate from the flesh and rise. Scoop them out with a slotted spoon. Count them if you like: 1 kg of damsons has roughly 60 to 80 stones.

Add the sugar and stir over low heat until every grain has dissolved. Do not let it boil until the sugar is dissolved, or the jam can turn grainy.

Turn the heat up and boil hard, stirring occasionally, for 8 to 12 minutes, until the jam reaches 105°C (221°F) on a thermometer.

Test the set: spoon a little jam onto a cold plate, wait 1 minute, then push it with your finger. If it wrinkles, it is ready. If not, boil for 2 more minutes and test again.

Take off the heat, stir in the butter to clear any foam, and let it stand for 10 minutes so the fruit is evenly spread.

Ladle into the hot sterilised jars, leaving about 1 cm headspace, wipe the rims and seal. For long-term pantry storage, process in a boiling water bath (see notes).