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Heat the oven 170C/150C fan/gas 3. Put the flour, butter and sugar into a mixing bowl. Use your hands to combine the ingredients until the mixture looks like breadcrumbs, then squeeze until it comes together as a dough.
On a lightly floured surface, use a rolling pin to roll out the dough to ½ cm thick. Cut the dough into fingers and place on a lined baking tray. Use a fork to create imprints, then sprinkle with the remaining caster sugar.
Chill the dough in the fridge for 20 mins, then bake for 15-20 mins until golden brown. Remove the shortbread fingers from the oven and leave to cool on the tray for 10 mins.