To make the brittle: Gently heat butter and sugar in a small pan until the sugar dissolves. Add the almonds and heat briefly. Line a plate with greaseproof paper or greased foil and spread mixture over it. Let set and then crumble.
Work the yolk, butter, sugar, vanilla sugar, flour, baking powder and brittle into a dough. Chill for 30 minutes.
Preheat oven to 180 C / Gas 4.
Roll dough out thinly and cut out small biscuits. Brush with beaten egg white and transfer to a parchment lined baking tray. Bake in preheated oven for 10-12 minutes or until golden brown.