Wash and stem the mint leaves, reserving 6 whole sprigs for garnish. Place the mint leaves in the bottom of a sturdy pitcher and add the sugar. Mash the mint leaves and sugar together, using the tip of a wooden spoon -- this extracts the mint oils. Add the lime juice and rum and stir until the sugar is dissolved. The recipe can be made several hours ahead to this stage. However, expect some discoloring in the mint leaves if you make the mojitos more than 20 minutes ahead.

Just before serving, add the club soda and gently stir to mix. Partially fill 6 highball glasses with ice. Add the mojito mixture and garnish each glass with a sprig of fresh mint.