1 double shot espresso (about 2 oz.), room temperature
1 ounce bourbon or rye whiskey (optional)
¼ ounce simple syrup
Dash of Peychaud’s bitters
1 1″ piece lemon peel
Combine espresso, bourbon, if using, simple syrup, and bitters in a cocktail shaker and fill with ice. Stir until outside of shaker is frosty, about 30 seconds; strain into a lowball glass filled with ice. Twist lemon peel over cocktail to release oils, then rub over the rim of glass; discard peel.