Using a Microplane grater, finely zest the orange and 1 lemon into a saucepan. Squeeze in the lemon juice and add the liquid glucose, sugar and orange juice.
Crush and add the juniper berries, then slowly bring to the boil over a medium heat. Simmer for 2 to 3 minutes, then remove and leave to cool completely.
Pass the mixture through a sieve and transfer to a wide, freezer-proof container, so it’s about 1 to 2cm deep (this will help it freeze quicker).
Pop in the freezer for 1 to 2 hours, forking through every hour until all of the mixture has crystalised.
To make the shortbread, gently beat the butter and sugar until pale and fluffy. Add the flour, cornflour and 1 pinch of fine sea salt, and mix into a dough. Wrap in clingfilm and leave at room temperature for 1 hour.
Roll out the dough between 2 sheets of baking paper to 5 to 6mm thick. Discard the top sheet of paper and cut the dough into rectangles (it should make about 8 biscuits).
Prick them with a fork, then slide the shortbread on its paper onto a baking sheet and chill in the fridge for at least 30 minutes.
Preheat the oven to 160ºC/325ºF/gas 3.
Bake the shortbread for 15 to 20 minutes, until golden. Remove, sprinkle with extra sugar, then leave to cool on a wire rack.
