According to cocktail historian David Wondrich, mint juleps were originally made with cognac. Bourbon was probably adopted as a substitute by Southerners after the Civil War.
2 tbsp. superfine sugar
10 spearmint leaves,
plus extra for garnish
1–2 tbsp. Simple Syrup
1/2 cup Kentucky bourbon
1. 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.
2. 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.