Combine the whiskey, simple syrup, and bitters in a mixing glass with ice. (If using sugar cubes, muddle the sugar and bitters and the smallest amount of water if necessary to create a syrup, then add the whiskey.)
Stir to chill, and adjust the syrup to taste. Rinse an Old Fashioned glass with the Herbsaint and discard the Herbsaint. Strain the cocktail into the absinthe-rinsed glass.
Flame the lemon “coin” over the glass and discard.