Muddle the orange slice, 2 cherries, bitters and sugar if using in a large liquid measuring cup until syrupy. Add in the bourbon and a few ice cubes and stir vigorously for 20 seconds to chill the drink and dilute the bourbon. Fill 2 rocks glasses with one large cube. Strain the drink into the glasses. Squeeze an orange peel above each glass to release the oils, then add to the glass along with 1 or 2 cherries.