Beat the double cream using an electric whisk until it holds its shape. Keep beating on a lower speed and gradually pour in the condensed milk and vanilla extract, then slowly add the lemon zest and juice. Put the mixture in a shallow baking tin or container that will fit in your freezer.

Drop the jam all over the surface in little blobs using a teaspoon, then drag a skewer through the blobs to spread it out a little. Some jam will disappear from the surface but that’s okay. Cover the container and put it in the freezer for at least 3 hrs. This is a soft-set ice cream, so you don’t need to take it out of the freezer too far ahead of serving. Will keep in the freezer for up to three months.