In the base of a stand mixer, add softened brown butter, sugar and salt. Mix gently 3-4 minutes to combine.
Add egg yolk. Whisk gently to mix thoroughly into butter.
Add flour and one teaspoon of ground cinnamon to the wet ingredients in one go. Mix again gently until the dough comes together in a soft crumble.
Tip out dough on to a sheet of baking paper. Form into a log (about 20 centimetres in length). Roll up in baking paper and twist the ends like a bonbon. Place into the fridge for one hour, or up to 3 days to chill and firm up.
Preheat oven to 180ºC (160ºC fan-forced). In a small bowl, combine demerara sugar and remaining half teaspoon of cinnamon. Sprinkle on to a tray or plate.
Unwrap your shortbread dough. Brush the outside with egg white and place into the demerara sugar, rolling it around to coat the outside.
With a sharp knife, slice the log into 12 biscuits, approximately 0.75cm thick. Place the biscuits on two lined trays.
Bake for 22 minutes. Once cooked, remove and leave to cool on the trays. Keep in an airtight container for up to five days.