For dark liquors, I prefer a darker simple syrup (as opposed to plain white granulated sugar and water). Demerara cane sugar is a large-grained, raw sugar with its origins in Guyana (a colony formerly called Demerara). Unlike brown sugar, which is refined white sugar with molasses added, Demerara sugar has a natural caramel-like flavor, which really elevates any cocktail to which it is added.
Here is how I like to make it:
In a small pot or saucepan, combine 2 cups of demerara sugar and 2 cups of water (for a 1to1 solution). Bring up to medium heat while stirring constantly but not bringing to a boil, until the sugar is completely dissolved. Add a teaspoon of pure vanilla extract. Lastly, add a tablespoon of vodka to kill bacteria and increase the shelf life of the syrup. Store the syrup in the refrigerator.