Chill a rocks glass. Meanwhile, in a mixing glass three-quarters filled with ice, mix the rye, simple syrup and bitters and stir until chilled, about 30 seconds.
Pour the Herbsaint into the chilled rocks glass and rotate until the liqueur coats the inside of the glass. Strain the contents of the mixing glass into the rocks glass. Rub the lemon twist along the rim of the rocks glass, twist over the drink and discard.
To make simple syrup, warm 1 cup sugar in 1 cup water in a saucepan over low heat until dissolved. Cool to room temperature before using. (There will be extra syrup; refrigerate if not using immediately.)