Preheat the oven to 180C/350F degrees.
Cream the butter and the erythritol together.
Add the egg yolks and mix well.
Add the baking powder, coconut flour, salt and cocoa powder. Mix well.
Whisk the egg whites until stiff peaks.
Gently fold the egg whites into the cocoa mixture and blend thoroughly.
Grease and line a rectangle baking tray with parchment paper.
Spoon the cake mixture into the tin and smooth out evenly.
Bake for 20 minutes until firm.
Remove from the oven and allow to cool.