Blitz the mango, 50g sugar and lime juice until smooth. Transfer to a pan, add the egg yolk and set over a medium heat. Whisk for 6-8 minutes, or until thickened. Beat in 10g butter. Cool.
Preheat the oven to 140°C, gas mark 1. Grease and line a 20cm square tin with baking paper.
Beat the eggs and remaining sugar in a bowl, until pale. Add the remaining butter, fold in the flours and lemon zest, then tip into the tin. Bake for 45 minutes, until just firm. Leave to cool completely.
Cut the sponge into 16 squares and make deep incisions in one side of each square. Pour the curd into a piping bag and use to fill the incisions. Freeze for 30 minutes.
Put the chocolate in a bowl. Warm the milk and pour over the chocolate, stirring to melt. Stir in the icing sugar. Tip the coconut into a tray.
Coat the lamingtons in the chocolate, then carefully dredge in the coconut.