Preheat your oven to 180C / 350F / 160C fan forced and line a lamington tin (approx 22cm x 33cm) with baking paper.
Sift together the flour, baking powder and salt and mix well.
In a microwave safe bowl, combine the butter and milk and melt in 30 second increments, stirring well between each. Add the vanilla and set aside.
In a new bowl, whisk together flour, cornflour, baking powder and salt until well combined.
Beat the eggs and egg whites together in a large bowl or the bowl of a stand mixer, until very thick pale ribbony.
While beating on low slowly pour in sugar into eggs, and beat a further 2 minutes,
Use a balloon whisk and a folding action to fold in half flour at a time until combined.
Pour in the milk mixture and use folding action again with balloon whisk until lump free. Don’t over mix and don’t whisk - just fold.
Pour into the prepared tin then tap lightly on the bench top 3-4 times until any large bubbles have disappeared.
Bake 20-23 minutes just until a toothpick comes out clean. Don’t check until at least 18 mins. Be careful not to overbake.
Let cool in tins for 10 – 15 minutes. Sit a large baking tray over the top and flip so the cake comes out. Repeat using a wire rack, so the cake is now right way up on a wire cooling rack.
Let it cool completely before cutting into 20 even squares.
FOR THE GLAZE
Combine the icing sugar, cocoa and boiling water in a deep bowl and mix well to a syrup consistency.
Place ⅓ of the coconut into a separate bowl. Set two wire racks over baking paper lined trays.
Using two forks, dip a square of sponge into the chocolate glaze turning to coat well. Let it drain a little then transfer to one of the wire racks. Repeat with 2 more squares.
Take the first square and use clean forks to roll it in coconut then transfer to the second tray to set. Repeat 2 more times.
Now repeat the process with the whole batch, replenishing the coconut as required.
For best results you should always weigh ingredients like flour and sugar. Kitchen scaleslike these are relatively cheap but if you can’t weigh the ingredients, use the spoon and level method (don’t scoop).
This recipe can be halved and made in an 8x8 inch square baking tin or, if you don't have a lamington tin make the full recipe in two square tins.