Preheat the oven to 180C/350F degrees.
Grease and line a cake tin (or tins) with parchment paper.
Beat the egg yolks with the erythritol until combined.
Add the coconut flour, lemon juice, lemon rind and baking powder. Mix thoroughly.
In another bowl, whisk the egg whites until stiff and firm.
Gently fold the egg whites into the cake mixture and combine well.
Spoon the cake mixture into the cake tin evenly.
Bake for 20 minutes until the sponge is firm.
Remove from the oven and allow to cool.
Slice in half (if you are using just the one tin)