Preheat oven to 350°F and spray a 13-inch by nine-inch baking pan with cooking spray and line the pan with parchment paper. Stay the pan once more with cooking spray.
Place a small saucepan over medium heat and add butter and milk. Stir until the butter has melted and set aside to cool.
Add flour and baking powder to a small mixing bowl, stir to combine, and set aside.
Add eggs, salt and sugar to a stand mixer fitted with the whisk attachment or, using a handheld mixer and a large mixing bowl, whisk on high until the mixture is thick and pale in colour, about five minutes. Reduce the speed to low and slowly add the flour mixture. Increase the speed to medium, add vanilla and cooled milk mixture, and beat until fully incorporated.
Add batter to the prepared baking pan and bake for 20 minutes or until a toothpick inserted into the middle of the cake comes out clean. Let cool completely.
Once the cake has cooled, cut it into 24 equal-sized (to the best of your ability) squares, add them back to the pan, cover them with plastic wrap and freeze for one and a half hours. The frozen cake will help tremendously when you coat them in chocolate and coconut.
While the cake is in the freezer, you can make the chocolate coating. Add all the ingredients to a mixing bowl and gently whisk until fully incorporated.
Set yourself up a coating station; the bowl of chocolate, a large bowl of coconut and a baking sheet lined with parchment paper and fitted with a wire rack.
To assemble very gingerly, add a frozen piece of cake to the chocolate mixture, and using a fork, turn to coat. Transfer the chocolate-coated cake to the coconut and gently coat in the coconut. Place the lamingtons onto the prepared baking sheet to set. Enjoy!
TIP: Cut the lamingtons in half for a slightly elevated version and fill them with your favourite jam and/or sweetened whipped cream.