Preheat the oven to 300F and place butter in a rectangular baking pan. In a medium mixing bowl, mix all the solid ingredients with a spoon: almond flour, coconut flour, shredded coconut, baking powder, lemon zest, and sweetener.
With the blender, mix the butter and the eggs and add the mixture of flour little by little until a homogeneous mixture is obtained.
Put in the mold and bake for about 30 minutes. Once cooked, remove from the oven and allow it to cool.
For the topping, mix all the ingredients in the mixer and cover the cake with the topping when the cake is cold. You can sprinkle extra lemon zest and unsweetened shredded coconut on top to decorate.