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Heat oven to 200C/180C fan/gas 6. Mix the butter with 50g of the sugar and the vanilla. Mix the flour, ground rice and ½ tsp cinnamon, then mix this into the butter mixture – you may need to finish with your hands. Roll out to just under 1cm thick between 2 sheets of baking parchment, then lift the bottom sheet of baking parchment onto a baking tray. Bake for 10-15 mins until golden and crisp looking.
Meanwhile, mix the remaining sugar with the remaining cinnamon. Remove the shortbread and leave to firm up for 1-2 mins, then slice into long shard shapes with a knife. Discard trimmings (or nibble them!), and scatter the biscuits with the cinnamon sugar while still warm. Gently toss to coat, then set aside until cool. Cover until ready to serve.