• Place a few small plates into the freezer (you'll use these for the 'wrinkle test' later on).

Sterilise the jars:

• Clean the jam jars and their lids thoroughly in very hot, soapy water and rinse well with hot water. Set the lids aside somewhere clean to dry. Place the jars (but not the lids) upside down on a baking tray and into the oven for 5-10 minutes until they're dry. Leave them in the switched-off oven so they stay warm as you make the jam.

Simmer the damsons:

• Place the damsons and water into a large pot. Cover with a lid and bring to a simmer. Simmer for 15-25 minutes or until the damsons are soft and starting to break down (the time will depend on their ripeness & if they were frozen or fresh). I like to give them a gentle mash in the pot as they simmer to help release the pits from the fruit. Remove from the heat.
  1.5 kg damsons, fresh or frozen, 150 ml water
  

Pass through a colander to remove the pits:

• Place a colander over a clean, large pot. Pour the damsons & any liquid from their pot into the colander. Use a silicone spatula or large spoon to push the damson flesh through the holes of the colander. Keep pressing and smushing the fruit against the sides and base of the colander until you think no more puree can pass through. The damson pits & much of the damson skin will be left in the colander which you can discard (or see notes below for how to use them to make damson gin).
  

Cook damson puree with sugar:

• Add the sugar to the large pot of damson puree and stir together. Place over a medium heat on the stove and stir often (this helps the sugar dissolve & stops it catching & burning) until the mixture is gently bubbling.
  600 g granulated sugar
  
• Turn the heat up so the jam starts to boil - at first there will be some light foaming on the surface but this will mostly subside & larger, glossy bubbles will begin to form as the jam thickens. Keep cooking it, stirring occasionally, until the jam passes the 'wrinkle test' (see next step) - around 12-20 minutes of boiling time.
  

The Wrinkle Test!

• Remove one of the small plates from the freezer. Take a small spoonful of jam and pour onto the cold plate. Let it sit for 30 seconds then slowly push your finger through it.
  
• The jam is ready when (A) the surface of the jam has a 'wrinkly' texture as you're pushing your finger through it and (B) the resulting line you've made in the jam remains there.
• If the jam doesn't pass the wrinkle test, cook it for 2 more minutes and try again with a fresh plate from the freezer.

Transfer to jam jars:

• Pour the hot jam into the prepared jam jars - for this quantity of fruit I get around 3 Bonne Maman jars worth (each with a 370g capacity). You'll want to pour the jam so it's roughly 1 cm from the top. Screw on the lids tightly then invert the jars for 1 minute. Flip them right side up and leave them alone for 24h. to cool and set.
  
• Store the jam in the jars somewhere cool & dark for up to a year. Once opened, keep in the fridge.