Preheat oven to 160°C (140°C fan forced) and lightly grease a lamington pan (23cm x 33cm) and line with baking paper.
Pour boiling water into a small saucepan, about a third full and sit a small bowl on top. Add Haigh’s 70% cocoa dark chocolate pastilles and stir occasionally, until chocolate melts.
Meanwhile, using an electric mixer, beat butter and sugar together for 5 minutes or until light and fluffy. Add eggs one at a time, beating well between each addition. Add flour to the egg mixture and using a large metal spoon gently fold together. Add cooled chocolate, again folding to combine.
Spoon mixture into prepared tin and bake in preheated oven 30 minutes, or until a skewer comes out of the centre clean. Remove from oven and allow to cool in tin for 5-10 minutes, before turning onto a wire rack to cool completely.
For the chocolate glaze, place Haigh’s dark chocolate pastilles, milk and water together in a mixing bowl, over a saucepan of simmering water. Stir together until chocolate melts and mixture becomes smooth. Sift the cocoa and icing sugar over mixture, stirring to combine until mixture is smooth. Remove from heat and allow to cool for 10 minutes.
Place coconut in clean mixing bowl ready for dipping lamington in.
Cut cooled sponge into 20 equal squares and dip each cake square in chocolate glaze and coconut. Place on a wire rack to set.