Muddle one sprig of mint with simple syrup in the bottom of a mixing glass. Add spirits, lime juice and egg white. Dry shake (without ice) 10 seconds. Add ice and shake vigorously to chill and emulsify. Strain into a chilled cocktail coupe. Garnish with the expressed oil of a lime zest, then discard the zest. Drop 8 drops smoked salt bitters (or salt tincture) on the surface of the drink. Garnish with remaining mint sprig.