Finely chop the lemongrass into small pieces or finely grind in a food processor or mortar and pestle.
Pour the milk into a large, heavy-based saucepan, add the lemongrass, and bring to a boil. Remove from the heat and leave to infuse for 1 hour.
Break the chocolate into cubes and melt in a bainmarie or microwave. Soften the gelatin in a small bowl filled with cold water and stir it into the chocolate until dissolved.
In a medium bowl, whisk the egg yolks and sugar. Semiwhip the heavy cream in a separate bowl. Whisk the egg-sugar mixture into the milk. Return to low heat, stirring continuously to slowly cook the egg until the mixture has slightly thickened. Remove from the heat and stir in the melted chocolate. Leave to cool slightly.
Pass the mixture through a sieve to remove the lemongrass. Allow to cool completely.
Slowly fold in the heavy cream (do not overwhip the cream or it will separate). Pour the mousse into molds (see Note) and leave to set in the refrigerator, about 2 hours.
Just prior to serving, make the sauce, juicing the lemons into a small heavy-based saucepan. Reduce by two-thirds over a low heat. Add a little sugar to taste. Thicken with cornstarch as desired.
To serve, turn each mousse out onto a plate. Garnish with caramelized lemon rind and drizzle with sauce.
The mold used in this recipe are 3 inches in diameter and 1 1/2 inches high. They can be either stainless steel, ceramic, or plastic, but not aluminum. Remember that the finer you chop the lemongrass, the more flavor the mousse will have.
Recipe adapted from Lemongrass and Lime (10 Speed Books, 2001) by Mark Read and Christian Benians