Pre-heat oven to 180C/350F degrees
Line a 8-inch square baking tin with parchment paper.
Whisk the butter with the monkfruit sweetener until creamy.
Add the egg yolks and vanilla and continue to whisk.
Add the coconut flour, salt and baking powder and mix well.
Add the water if the mixture is too stiff.
In a separate bowl whisk the egg whites to stiff peaks.
Gently fold in the egg whites and mix until combined
Spoon into the tin, evenly smooth out the cake mixture.
Bake for 25 minutes until the sponge is golden and firm.
Remove from the oven and set aside to cool.