Mix the butter, caster and icing sugar, egg yolks
and vanilla with a wooden spoon
until creamy, then mix in the flour
in 2 batches. Roll out to a 20 x 30cm
rectangle on a sheet of baking
parchment. 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. Roll up
from one of the 20cm sides using the
parchment to help, wrap in the parchment
and chill for 30 mins, or up to a day.
Heat oven to 200C/180C fan/gas 6.
Thinly slice into about 20 biscuits, scatter
with a pinch more sugar, then bake on
baking sheets for 10-12 mins until golden.