Muddle a sugar cube and bitters with one bar spoon of water at the bottom of a chilled rocks glass. (If using simple syrup, combine bitters and one bar spoon of syrup.) Add rye or bourbon.

If you don’t have a muddler, don’t sweat it; just use the handle of a wooden spoon. Combine the sugar cube, water, and bitter in the glass and muddle (i.e., crush) until the sugar is dissolved.