Below is the inggredients (not all of them), that is how Gula jawa looks like, it usually in cylinder. The other one is brown sugar.

Heat the wok, add the oil. Meanwhile, Blends all the inggredients with blenders (I like to do this till smooth, some people prefer this not too smoth, up to you). Start cooking. As guideline, the paste should be swimming in oil (see picture), This will take about an hour-1.5hours in medium heat. Stirred occasionaly (I start with 30 minutes in timer, and gradually move to 15 minutes to stirred). sometimes add more oil in the process.

After 1 hour this how it should looks like, darker and a bit shiny. This is when you taste and add any that missing (su gar, salt, etc) the paste should gradually lost all the water and you can see the oil start separating. It usually took about 1.5hr for me. And it will last me for more then 2 month in fridge.

Note
1. I used thai chillies, this is optional
2. Sugar type can be anything from as simple as white sugar, originally this used Gula jawa which means coconut sugar (palmyra palm) or we also used palm sugar (this can be found in asian supermarket). This can be subtitute with brown sugar. Fyi: This is not the same as jaggery (as this taste more sour, but you can of course try using it).
3. You can freeze this, I prefer to freeze in batches, that I know I would finish in a couple of weeks.

If you dont want too spicy, dont use seed of chillies and use more tomatoes.