Put a mixing bowl (preferably glass) big enough to hold a batch of ice cream in the freezer.
Mix the peanut butter well to re-integrate any oil that has started to separate out to the top of the jar.
Mix the peanut butter with the sugar and egg - this can be hard work. Then mix in the cream.
Pour into the can and top up with milk. Freeze.
Once the ice cream is frozen, transfer it to the cold mixing bowl and add the frozen jam.
Turn the ice cream and jam together, very lightly, at most 5-6 turns with a spoon to make a swirl, too many and it will just dissolve into it.
Ideally, then put the ice cream into the freezer to firm up, we just served it straight off.
We used St Dalfour jam which is sweetened with grape juice rather than sugar, but any jam would be fine. This batch used strawberry jam which was good but I'm curious to try raspberry - it'd be interesting to see if the seeds in the raspberry detracted from the smoothness of the ice-cream.
The choice of peanut butter is really important, some "smooth" peanut butters can still be quite grainy. Also avoid those that have palm oil or vegetable oil added to them - it gives a weird waxy quality to the ice-cream. We used Suma because its really smooth and 100% peanuts, plus and I really like the flavour.