Muddle 20 mint leaves with 2 tablespoons raw sugar in cocktail shaker.
Once the mint is broken down, add the juice of 1 lime, 1/2 cup light rum and fill the shaker with ice.
Run the lime wedges around the rims of two collins or highball glasses.
Dip the rims into the raw sugar spread on a plate.
Shake the cocktail until ice cold and then carefully divide it between the glasses, minding the sugar rims.
Add more ice if necessary.
Top each glass with ginger beer.
Garnish with fresh mint.