Beat 150g (5oz) softened butter with 75g (3oz) caster sugar until pale and fluffy. Then beat in 1 egg yolk, 1 teaspoon vanilla extract and 3 tablespoon milk. Sift in 250g (9oz plain flour and mix to a soft dough. Chill for 15 minutes.
Roll out the dough on a sheet of baking parchment to form a rectabgle about 30 x 20 cm (12 x 8in).
Brush the surface of the dough generously with beaten egg white. Mix 4 tablespoons demerara sugar with 1 tablespoon ground cinnamon and sprinkle over the surface of the dough.
Roll up from the long side like a Swiss roll. Wrap in the parchment and chill for 30 minutes.
Preheat the oven to 180℃ (160℃ fan)/350°F/gas mark 4.
Cut the dough into 5mm (¼in) slices and place on a baking sheet allowing room for the cookies to spread during baking.
Bake for 10–12 minutes until pale golden. Cool for a few minutes on the baking sheet before transferring to a wire rack to cool completely.