Level off the bottoms of the oranges so that they can sit upright. Sit on a baking tray and transfer to the freezer with the tops replaced.
Meanwhile, prepare the granita by pressing the orange flesh through a sieve and collecting the juice in a bowl. Add the 1litre of fresh orange juice to it and stir well. Add the caster sugar and stir until dissolved. Add the Cointreau and stir well.
Strain the mixture into a shallow roasting tray and chill for 1 hour. Remove after 1 hour and scrape the frozen mixture with a fork to create a slushy, granular texture. Return to the freezer for another hour and repeat the process of breaking up the ice with a fork after an hour.
Return for a further hour until frozen and and scoop into the frozen empty orange shells. Serve immediately, with the tops replaced on top.
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