Preheat your oven to 180 °C / 355 °F. Line your baking pan (35 cm x 25 cm / 14-inches x 10-inch ) with parchment paper. Set aside until needed.
In a bowl stir to combine all-purpose flour, a pinch of salt and baking powder. In a separate bowl (or in a bowl of a stand mixer) whisk together eggs, sugar and vanilla using an electric mixer or stand mixer. Beat for about 5 - 8 minutes or until the mixture has tripled. Add the melted (but cool) butter. Give it a good mix to combine. While beating on low-speed alternate dry ingredients and milk to egg mixture in three additions until well incorporated. Pour the mixture into the prepared baking pan. Spread evenly.
bake the sponge cake
Place the baking pan into a preheated oven. Bake for 30 minutes at 180 °C / 355 °F. Transfer the baked sponge cake to a wire rack to cool.TipHow to make sure the sponge cake is baked? Insert a skewer into the center of the cake. It should come out clean. If it doesn't, bake for another 5 minutes or so.
freeze the sponge cake
Wrap the baked (and cool) sponge cake in cling film. Place in the freezer for 30 - 60 minutes. This is a very important step and we definitely don't recommend skipping it. If you would dip freshly baked (unfrozen) sponge cake into the chocolate it would start breaking apart. Also, it's easier to cut the sponge cake on even squares once the cake has frozen.
Bring the milk to a boil in a saucepan. Roughly chop your chocolate (or use chocolate chips). Place the chocolate in a large bowl. Add the unsweetened cocoa powder and butter. Pour the milk over the mixture and set aside for about 5 minutes for the chocolate to melt. Using an immersion blender or whisk mix until you get a nice, silky smooth chocolate mixture. Add desiccated coconut in a separate bowl.
Remove the sponge cake from the freezer, remove the cling film. Optionally cut the edges off (to get a more even rectangle). Using a serrated knife, cut the sponge into 30 squares (approx 5 cm x 5 cm / 2-inches x 2-inches). Using two forks, dip one sponge cake square into the chocolate coating mixture, making sure that all sides are coated. Allow any excess chocolate to drain off. Then roll the chocolate-coated sponge square in the desiccated coconut, and place the lamington on a wire rack to dry. Repeat the process until all lamingtons are coated. Serve immediately or store in an airtight container for up to 5 days.TipDon't leave the sponge cake squares in the chocolate mixture for too long or they will start falling apart.