Add mint leaves and simple syrup to mixing glass and muddle until fragrant, about 30 seconds. Add bourbon, then fill glass three-quarters full with ice. Stir until mixture is just combined, about 15 seconds.
Double-strain cocktail into chilled old-fashioned glass or julep cup half-filled with crushed ice. Top with additional crushed ice to form mound above rim of glass. Garnish with mint sprig and serve.
Ingredient Note: Add a Twist
