Squeeze the lime for juice and pour into a tall glass. Add the mint and sugar, then muddle (gently pound) the mixture. Add the rum and fill with ice to the top of the glass. Finish by pouring Singha beer to fill the glass.
Serve with a garnish of lime and extra mint.
If you have some leftover Singha from the bottle, you can serve alongside your Mojito as a chaser.
Recipe contributed as part of the Senses of Thailand promotion at Selfridges.