Heat the mixture until boiling, stirring occasionally.
Cook for another minute or two, until the sugar is fully dissolved.
Cool and store in a covered jar in the refrigerator.
Demerara sugar can be found in some high-end grocery stores and specialty cookware stores. You can also buy it online at Amazon. If you can't find or don't have Demerara sugar you can substitute raw or turbinado sugar.
You can store the syrup in a covered container in the refrigerator for up to two weeks.
For longer storage, add 1 teaspoon of vodka to the syrup and stir. The alcohol will help inhibit mold formation.