1. Preheat oven to 190°C conventional or 170°C fan forced. Grease a 30cm x 29cm lamington tin. Line the base and sides with non-stick baking paper, allowing it to overhang slightly.
2. Sponge cake Sift the combined flours three times into a bowl. Using an electric beater whisk eggs in a large clean, dry bowl until thick and pale. Gradually add Chelsea Raw Caster Sugar, 1 tablespoon at a time, whisking well after each addition until mixture is thick and sugar dissolves.
3. Sift the combined flours over the egg mixture. Combine butter and boiling water and pour down the side of the bowl. Use a spatula to gently fold in until just combined.
Pour mixture into the prepared tin. Bake for 20 -25 minutes or until a skewer inserted into the centre comes out clean. Turn cake onto a wire rack to cool. Place in an airtight container. Cake is best made the day before cutting.
4. To make lamingtons Cut sponge cake into 18 pieces. Put coconut into a wide bowl. Make icing by combining chocolate and water in a heatproof bowl over a saucepan of simmering water. Stir until chocolate has melted and mixture is smooth. Remove from heat.
5. Sift Chelsea Icing Sugar and cocoa over chocolate mixture, stir until smooth. Add milk and mix until well combined. Work quickly as chocolate sauce will thicken on cooling.
6. Using a large fork, dip sponge cake pieces into chocolate mixture, drain slightly then roll in coconut. Leave to set on a tray.