For the panna cotta, heat 400ml/14fl oz of the cream with the lemongrass and sugar in a saucepan.
Bring to a boil, stirring occasionally until all the sugar has dissolved, then reduce the heat and simmer until the volume of the liquid has reduced by half.
Remove from the heat and allow to cool. Once cooled, pour the lemongrass cream through a fine sieve into a bowl.
Heat the coconut milk in a separate saucepan until just simmering, then remove from the heat.
Drain the gelatine leaves and add to the warm coconut milk. Stir until dissolved then pour through a sieve into the lemongrass cream. Allow to cool.
Whisk the remaining cream and coconut liqueur in a bowl until soft peaks are formed when the whisk is removed. Fold into the cooled cream until totally incorporated and pour into four ramekins or moulds. Set asid in the fridge for at least two hours until set.
Meanwhile, blend the mango pieces to a fine purée in a small blender. Pass the mango purée through a sieve into a bowl. Set aside until serving.
For the coconut meringue, preheat the oven to 100C/200F/Gas ¼.
Place the egg whites into a clean glass bowl and whisk until soft peaks form when the whisk is removed.
Whisk in the caster sugar a spoonful at a time until the mixture is glossy and stiff peaks form when the whisk is removed.
Line a baking tray with parchment paper, or use a silicone mat, and place eight spoonfuls of meringue mixture onto it. Scatter the toasted coconut over the top.
Bake in the oven and for 1 ½ hours.
Remove the meringues from the oven and set aside to completely cool.
To serve, dip the panna cotta into a bowl of hot water briefly or slide a knife around the outside edge carefully. Spoon the mango purée onto the plate. Turn the panna cotta out onto the purée and set the meringue alongside.