Preheat oven to 350 degrees F.
Line an 8 inch spring form pan with parchment paper, set aside.
In a bowl, mix together the almond flour, erythritol, and lemon zest. Add the egg yolks and mix until a tough dough form.
In a separate bowl, add the egg whites and beat until soft peaks are achieved.
Add half of the beaten egg whites to the almond flour dough, and mix well until the dough becomes looser.
Next, add the remaining beaten egg whites, and gently fold the mixture, being careful to not knock the air out of the batter.
Transfer the batter to the lined baking pan. If desired, sprinkle the coconut chips or sliced almonds on top.
Bake for 25-30 minutes, or until golden around the edges and tender in the center.
Remove the cake from the oven and allow to cool for 15 minutes.
Sprinkle with powdered sweetener and serve.