Preheat the oven to 200C/400F degrees.
Blend together the butter, eggs, erythritol, lemon juice, lemon rind and vanilla.
Add the coconut flour, baking powder, salt and whisk until there are no lumps and it is a smooth batter.
Pour into a parchment paper lined baking tin. I use a 8 inch square tin.
Bake for 20 minutes and allow to cool.
Blend the topping ingredients until smooth. I used a hand blender but feel free to do an upper arm workout and try it with a fork/spoon!
Slather the topping over the cake.
Cut into 8 slices or smaller squares.