In a bowl, combine coconut flour, collagen powder, erythritol and baking powder.
Whisk egg yolks with melted butter and vanilla extract, then add mixture of dry ingredients and combine.
Beat egg whites until bubbles start to form. Add a pinch of salt and lemon juice and beat until the whites begin to increase in volume. Add lemon zest and beat until stiff peaks form.
Fold beaten egg whites in egg yolk mixture until blended through.
Grease a loose bottom 9-inch cake tin or any standard springform pan you have with butter then line the bottom with baking paper or use silicone cake mold.
Pour the batter to the cake tin and bake for 25 minutes or until golden on top.
Let the cake cool completely before removing it from the pan and adding your favorite topping or icing of your choice (optional).