Turn oven on to 190°c bake and line a 20cm square tin with baking paper.
Starting with the base, melt butter in a large pot then set aside to cool slightly.
When the butter has cooled, add sugar, egg, vanilla and mix until smooth.
In the same pot, sift in the flour, baking powder, cocoa and then add the coconut, mixing well to incorporate.
Pour base mixture into prepared tin, smooth and place in the fridge while you make the sponge.
Using a free-standing mixer beat the eggs, slowly adding the sugar in increments before adding the vanilla. Continue to beat egg mixture for approximately 10 minutes until pale and fluffy.
In a separate bowl, sift together the dry ingredients and melt butter in another small bowl, again setting aside to cool.
Add dry ingredients to egg mixture along with the cooled and melted butter. Fold through with a spatula until just combined, don’t over mix.
Pour cake mixture over top of the base, place tin in the centre of the oven and bake for approximately 50 minutes. After the first 30 minutes, remove the slice from the oven and cover with tin foil to avoid the top darkening too much. Return slice to the oven and bake for the remaining 20 minutes or until sponge springs back when touched or skewer comes out clean. Allow the slice to cool in tin.
While the slice is cooling, start on the icing. Place chocolate and water in a small pot over top of a larger pot of boiling water. Allow the chocolate to melt and continue mixing until smooth.
Remove melted chocolate from the heat and sift in the icing sugar and cocoa powder, mixing until it forms a paste. Add milk and vanilla and continue to mix until smooth.
Using a spoon, pour or drizzle the chocolate icing over top of the sponge. Making sure you create a good and even coverage.
Finally sprinkle plenty more dessicated coconut over top of the chocolate icing, just like a real Lamington. Place the slice in the fridge to cool and set completely before cutting with a shape knife. Enjoy.