1) Heat oven to 200C/180C fan/gas 6.
2) Mix the butter, caster and icing sugar, egg yolks and vanilla with a wooden spoon until creamy, then mix in the flour slowly.
3) Roll out to a 20 x 30cm rectangle on a sheet of baking parchment.
4) Mix the demerara sugar with the cinnamon, sprinkle all over the dough, then gently roll over a rolling pin again to press the sugar in a little.
5) Roll up from one of the 20cm sides using the parchment to help, wrap in the parchment and chill for 30 mins.
6) Thinly slice into about 20 slices, sprinkle a little more sugar over the tops, then bake on baking sheets for 10-12 mins until golden.