This smoky twist on a margarita is the house specialty at the historic Hotel Fauchere in Milford, Pennsylvania. The smoke comes from the mescal (made with fire-roasted cactus hearts). The brand the hotel uses is Sombra. Strega is an herb-flavored liqueur from Italy; agave syrup is a sweetener made from agave cactus and widely available in supermarkets.
Step 1: Chill a cocktail glass in the freezer or with ice.
Step 2: Combine the mescal, tequila, lime juice, Strega, and agave syrup in a bar shaker with 1 cup ice cubes. Shake hard and strain into a chilled cocktail glass. Add a dash or two of celery bitters and serve.
One good brand of Celery bitters is Frye Brothers.