Combine all ingredients in a thoroughly clean pot (when I make syrups or jams, I like to give my already clean pots a thorough re-washing just to make sure I don't have any residual grease or savory remnants left on the surface), give them a good stir, and put the pot on medium heat, stirring occasionally until all the sugar melts and just until all the basil wilts, not longer.

Remove basil, turn off the heat, cool the syrup completely, and store in a thoroughly cleaned and dried tightly lidded glass or plastic container in the refrigerator.