In a food processor, pulse the butter and flour with the ground almonds and sugar until well mixed. Next add the egg yolk, almond extract and vanilla extract and pulse until the dough sticks together and is smooth.
Rest the dough in the fridge for a minimum of 30 minutes.
Preheat the oven to 180°C (350°F) Gas 4.
Put the pearl/nibbed sugar and the chopped almonds in a bowl and combine.
Roll the dough into 4 logs of approx. 45-50 cm. Cut into pieces approx. 5 cm. (each about 20g).
Dip one side in the egg white and then press into the nut/sugar mixture, flattening the biscuits as you do so. Put on the prepared baking sheet, nut/sugar-side up and repeat for the remaining pieces.
Bake in the preheated oven for about 8–11 minutes until just browned at the sides. Transfer to a wire rack and leave to cool completely. Store in an airtight container.
