Place the butter, flour and lemon zest in the bowl of a food processor and blitz until it resembles breadcrumbs. Add the sugar, egg and rum and bring together to form a dough.
On a lightly floured surface, roll the pastry out and use it to line a 20cm straight sided tart tin, trim the excess pastry and chill for 30 mins.
Heat oven to 180 C.
Line the case with baking paper and baking beans and bake blind for 20 mins, then remove the beans and continue to cook for a further 20 mins until the base is biscuity. Remove from oven and reduce the temperature to 140 C.
Bring the coconut milk, cream, lemongrass, and rum to the boil in small saucepan. Beat the egg yolks with the sugar, then pour the hot milk and cream over, whisking as you go.
Carefully pour the custard into the tart case, and bake for 20 mins or until just set with the very slightest wobble in the middle. Remove from the oven then leave to cool completely.
To make the meringue, stir the sugar and water in a saucepan over a low heat until the sugar dissolves.
Cook on medium-high heat until the sugar syrup reaches 115 C.
While the syrup continues to cook, whisk the egg whites and cream of tartar until soft peaks form. When syrup is 121 C, slowly add it to the egg whites, making sure not to pour the syrup over the whisk.
Whisk for 10 minutes or until thick and cooled and then add the desiccated coconut and whisk one last time to incorporate. Spoon the meringue onto the baked pie.
Use a spoon to make large swirls and peaks in the meringue. Use a blowtorch to caramelise the meringue and then serve.