• Before you start, make sure your butter is softened and eggs are at room temperature. 
• Place the egg whites in a deep bowl. Add sea salt and citric acid and sweetener. Start beating them with an electric mixer and don't stop until they create a firm meringue. This depends on your mixer, maybe it will take from 7 to 15 minutes. be patient. 
• When meringue is firm, add powdered lemon zest and coconut flour. Now mix gently with a whisk. The meringue will fall a little, but don't worry. 
• Line a large baking sheet with a parchment paper and put the egg white mixture into a pastry bag and squeeze it to create round shapes. Make them approximately 4 cm in diameter and leave some space in between. For each pair, make a smaller circle which will go on the top. 
• bake them in the oven at 130ºC (260ºF) for 30 minutes. In the meantime prepare the cream. 

Filling

• Beat the softened butter with the sweetener and all the flavourings of your choice. This part is really important to be patient. Beat the butter for at least 10 minutes or as long as you need to get it fluffy and creamy. 
• Using a small whisk, beat the egg yolks and slowly add 100 ml of hot boiling water. Ass you are adding the water, whisk the eggs energetically.
• Add the egg yolk mixture to the butter spoon by spoon. Keep mixing at a high speed. As time passes the cream will become thick. 
• When the pastries are baked, let them cool down before you try to unstick them from the parchment paper. If they are breaking, then use a sharp knife to gently de-attach them from paper. 
• Place the filling cream into a pastry pipe with a star attachment. Fill the pastries with cream and be generous. I created 3 layers, you can make only 2.
• This butter based cream hardens (takes the icing texture) totally in the refrigerator, so make sure you have them at room temperature at least 15 minutes before serving. At room temperature, it becomes nice and creamy again.