Mix the lemon grass with the coconut milk, squeezing it to release the perfume. Leave to infuse for 30 minutes. Squeeze once more, strain and then discard the lemon grass.
Fill a wok or large saucepan with water and bring to the boil over a medium heat. Meanwhile, put the lemon grass and coconut milk mixture in a saucepan with the double cream, stir over a gentle heat without boiling for 3–4 minutes and set aside.
Place the egg, egg yolks, sugar and salt in a large heatproof bowl over the boiling water. Use an electric mixer to whisk the mixture for 3–4 minutes until frothy and thickened. Gradually add the lemon grass and coconut milk mixture and mix for another 5–6 minutes to make a thin cream. Leave to cool, pour into a plastic container and freeze for 3 hours or until half-frozen.
Take the ice cream out and 'comb' it with a fork once or twice during the freezing time. Cover and freeze completely. Remove the ice cream from the freezer at least 10–15 minutes before serving so that it becomes slightly softened. Serve on its own or with White Sticky Rice (see