Whiskey syrup is a combination of moistening syrup & whiskey used as a lubricant when making roll cakes, such as log cakes, to reduce cracking when you roll the cake.
Combine 1 cup sugar and 2/3 cup water in a small saucepan.
Cook over low heat, stirring gently, until the sugar dissolves. It does not need to come to a simmer to dissolve the sugar and absolutely should not boil.
Remove from the heat then stir in 1/3 cup whiskey.
It will keep a few weeks in the pantry or a few months in the fridge.