Preheat the oven to 175C/350F. Line and grease a 32cm x 27cm baking tray (a similar size will be fine. Just make sure you'll be able to cut at least 2 Iron Man templates out. The 3rd template will be made from the left over cake).
Place the egg whites into the clean bowl of a kitchen stand mixer fitted with a whisk attachment. Whisk the egg whites on medium/high until soft peaks form.
Slowly add the maple syrup, 1 tablespoon at a time, until large peaks have formed. Continue whisking on medium/high and add the vanilla extract.
Add the egg yolks, one at a time. Only add the next yolk when the previous one has been fully incorporated.
Whilst still whisking, pour the milk, juice and zest into the egg mixture.
In a separate bowl, lightly whisk the coconut flour and baking powder together. Ensure that the coconut flour has no lumps in it.
Turn the whisk to low and add the flour mixture. (If you keep it on high then you will most likely end up wearing the flour).
Once incorporated, return the whisk speed to medium/high to fully mix all the ingredients.
Pour the cake mixture into the prepared tin and bake for 18 minutes, or until cooked through.
Remove from oven and allow to cool in the tray.
Once cooled, remove the cake from the tray and place the template over 1 side of the cake. Cut out the template, and then repeat on the other side of the cake (this should result in 2 full templates). Keep the left over pieces to be used to 'construct' the 3rd layer of the cake.