In a medium bowl combine coconut flour, baking powder, and powdered erythritol.
Divide the eggs into two separate bowls. Beat egg yolks with an electric mixer with melted butter, vanilla extract and heavy cream. Add coconut flour mixture in and mix well.
In a separate bowl, beat egg whites until bubbles start to form then add lemon juice and beat again until it forms soft peaks. You know they are ready when their volume is increased and the texture becomes foamy.
Fold the egg whites into the egg yolks-flour mixture. Beat one more time until homogeneous.
Grease a small cake pan with butter then pour the cake batter into it.
Bake in the preheated oven for 25-30 minutes.
Remove from the oven and let it chill for a while before cutting.