Combine the rum, lime juice, simple syrup, and amoretto in a highball glass, add a handful of clean snow; stir to blend. Drop the spent lime into the drink.
Add more snow to the glass (fill to the brim). Insert a long bar spoon, holding it between your palms, and swizzle by sliding your hands back and forth until the glass is frosted.
Add a bit more snow; it should rise above the top of the glass mounded slightly. Top with the bitters.
Garnish with a mint sprig and a light dusting of confectioners’ sugar.