Heat 400ml cream, lemongrass, and 75g sugar in a saucepan.
Boil until sugar dissolves, then simmer until liquid reduces by half.
Cool the mixture and strain through a fine sieve into a bowl.
Heat coconut milk in a separate pan until just simmering.
Dissolve drained gelatine in warm coconut milk and strain into the lemongrass cream.
Whisk remaining cream and liqueur to soft peaks, fold into cooled mixture, and chill in moulds for two hours.
Blend mango into a fine purée and strain through a sieve.
Preheat the oven to 100°C.
Whisk egg whites in a clean bowl until soft peaks form.
Gradually whisk in 150g sugar until the mixture is glossy and stiff.
Place eight spoonfuls of mixture onto a lined tray and top with toasted coconut.
Bake the meringues for 1.5 hours.
Cool the meringues completely after removing from the oven.
Unmould panna cotta onto mango purée and serve with the coconut meringue.