Mix everything together in a small saucepan. Cook over medium heat and stir constantly to dissolve the sugar. Simmer until the sauce has thickened just enough to coat the back of a spoon, 10 minutes or so. Remove it from the stove and let cool for 10 minutes. Then transfer to a container or heat-proof mason jar. The sauce will thicken further once cooled. It’s ready to use now, or you can store it sealed in the fridge for up to 4 days.