Preheat the oven to 175C/350F.
Take a 20cm square cake tin and line with baking paper, making sure that 2 edges extend over the sides of the tin. A round 20cm/8 inch tin can also be used.
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 30-35 minutes or until cooked through.
Remove from the oven and allow to cool in the tray for 30 minutes.
After 30 minutes take a wooden spoon and use the handle end to make small indentations in the cake. The holes should go 2/3 of the way down the cake. Make 4 or 5 rows of 5 holes.
Allow to cool completely and do not remove from the tin.