Put sugar on a plate. Moisten the rim of a glass (if you have a silver julep cup, that's even better) with a damp towel.

Press rim of glass into some of the sugar to coat.

Put leaves and simple syrup into glass.

Crush leaves very gently with a pestle, just enough to bruise them.

Fill to the brim with crushed ice, then pour in bourbon to cover ice.

Stir one turn with a swizzle stick.

Garnish with a leafy sprig of spearmint.