Crush 12 leaves of mint and 2 stems and drop them into a collins glass. Squeeze 4 wedges of lime into the glass and drop them in too. Add a bar spoon of brown sugar and a splash of gomme (sugar syrup). Muddle until everything is mushed and smelling "minty limey". Add 2 measures (50ml) of Havana rum and put the bar spoon in the glass, flat side down. Whack in enough crushed ice to 3/4 fill the glass and mix thoroughly with the bar spoon under the ice. Remove the spoon and add a little more ice and top with soda water. Straw test it, if its a bit strong then add some lime cordial but ask the customer how they like it :)