Mix butter, sugar substitute, almond extract together (easiest done in food processor if you have one), then add egg yolks and the tbsp of egg white. Mix well, then add rest of ingredients EXCEPT for the powdered sugar sub. Wrap dough in clingfilm (saran wrap) and chill for an hr.
Preheat oven to 180C. Line a baking sheet with baking parchment.
Take walnut size pieces of dough and shape into crescents. Bake until lightly golden for 12-15 minutes. Gently coat in the powdered sugar substitute while warm and leave to cool completely.