Preheat the oven to 350 degrees. Grease and flour a square baking pan and set aside.
In a bowl mix the almond flour with the coconut flour and the baking powder.
Add the egg, melted coconut oil, lemon juice, vanilla extract, almond milk and maple syrup and stir until everything is well incorporated.
Add the beaten egg whites in 2 additions, carefully folding them in with the help of a spatula.
Fold in the blueberries and pour the batter into the prepared cake pan.
Bake for 35 minutes or until done.
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Notes
You can use any sweetener of your choice. I have also tested this recipe with erythritol and the result was just as good (used a little bit less than ½ cup)The egg whites make the cake fluffier so I definitely recommend folding them into the batter.You can use butter or the baking oil of your choice instead of coconut oil.