Grate the brown sugar lumps roughly. Put the sugar and water in a small pan and heat. Mix it up from the bottom while simmering over low-medium heat.
When it comes to a boil, turn the heat down to low. Simmer for 10 to 13 minutes or so while skimming off any scum until the syrup is reduced by about half, and it's done.
Story Behind this Recipe
In the summertime, I often want to eat kanten (agar-agar) jellies. I received lump brown sugar as a gift, so I referred to a cookbook from Okinawa to make this syrup.
When reducing the syrup, take care not to let it boil over. If you let the syrup burn, it will become bitter, so keep mixing as you simmer. For people who wrote in concerned about the syrup crystallizing while chilling, the syrup was OK for 10 days.