Chill a rocks glass and then rinse the inside with Herbsaint 100 (or absinthe if you don't have it).
In a mixing glass, combine the bitters, Cognac and syrup. Stir until cold. Strain into the Herbsaint 100 rinsed glass. Express the lemon oil from the lemon peel into the glass, wipe the rim of the glass with the lemon peel and then discard the peel.