How to Make It
Bring sugar and water to a boil in a small saucepan over medium-high, stirring occasionally. Boil, stirring constantly, until sugar dissolves. Remove from heat, and add mint leaves, lemon zest, and lime zest, gently pressing on the leaves with a spoon to release their oils. Let stand 10 minutes.
Pour mint mixture through a fine wire-mesh strainer into a bowl; discard solids. Cool completely, about 20 minutes.
Pour cooled liquid into the bowl of an electric ice-cream maker; prepare according to manufacturer’s instructions.
Scoop about 3/4 cup of the mint sorbet into each of 6 rocks glasses or sorbet cups. Top each with 1/4 cup bourbon; garnish with mint sprigs.