If using fresh ube, steam or boil until fully tender. Leave to cool, then peel and grate.
If using frozen ube, thaw to room temperature.
Add the milks, sugar, salt to the pan, and stir to dissolve the sugar.
Add the grated ube, and stir to combine.
Heat to medium-high, then blend with a stick blender, so the mixture becomes as smooth as possible.
Once blended, bring the mixture to a boil. Stir continually.
Once the mixture comes to a boil, reduce the heat to medium or medium-low and continue cooking for 40-60 minutes. Stir continually (or continuously if required) to prevent the jam from sticking & burning.
When the jam starts to thicken, whisk in the vanilla, lemon juice and butter.
If you want to add ube extract as well, do it now.
Cook until the consistency is that of a thick custard. You need to take it off the heat when it is thinner than you need, as it will thicken significantly as it cools.
If keeping as a spread, carefully spoon the mixture into glass jars while hot and put the lids on. Allow the jam to cool down to room temperature then refrigerate. The ube jam will last in the fridge for about 5 days, and about 3 months in the freezer.If making into a dessert, spoon the mixture into greased moulds and place straight into fridge.