Place mint and ¼ ounce simple syrup in a julep cup or an 8 to 10-ounce old-fashioned glass.
Gently crush leaves with a wooden muddler; working them up the sides of the glass.
Loosely pack glass with finely crushed ice; add bourbon.
Drizzle remaining simple syrup on top.
Garnish with a mint sprig which has been lightly dusted with castor sugar.