Preheat the oven to 175C/350F.
Take a 2 Lt Pudding Basin and a 6 inch cake tin. Place a small piece of baking paper on the bottom of the pudding basin and grease the entire tin. Line and grease the 6 inch tin as normal. For alternative tin choices see the notes above the recipe.
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, add the zucchini and pour the milk, lemon and zest into the egg mixture.
In a separate bowl, lightly whisk the dry ingredients 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 and fully mix all the ingredients for 20 seconds.
Fold the blueberries through gently.
Pour the cake mixture into the prepared tins and bake for 1 hour and 20 minutes (the pudding basin) and 30-40 minutes (for the 6-inch tin), or until cooked through. After 1 hour, cover the top of the pudding basin with foil to prevent the top of the cake from burning.
Remove from the oven and allow to cool completely in the tray.
When removing the pudding cake; run a knife or skewer around the edge of the tin before inverting it. The cake edges may be broken and ugly but that’s ok as it will all be covered in cream and frosting.