Preheat the oven to 160°C/fan 140°C/gas mark 3. Heat the cream and sugar to scalding point, not boiling, in a small saucepan. While still on the heat, whisk in the lemon zest and juice and bring to the boil for 30 seconds, whisking all the time.
Remove the pan from the heat and put to one side for the posset mixture to cool. When cool, pour into 4 small (150ml) glasses, then refrigerate until set. This should take around 30 minutes.
To make the shortbread, whizz the flour, butter and caster sugar together in a food processor, until the mixture comes together. Alternatively you can do this with your hands by rubbing the mixture together between your fingertips and then lightly pressing it together to form a ball – don’t overwork the dough at this stage. Turn out on to a sheet of non-stick greaseproof paper and roll out to ½cm thick using a floured rolling pin. Using a 5cm round pastry cutter, cut out rounds of dough and carefully transfer to a baking sheet lined with more greaseproof paper. Sprinkle with some extra caster sugar and bake in the preheated oven for 10–15 minutes or until lightly golden. Once out of the oven allow the shortbread rounds to rest for 10 minutes.
Serve the posset chilled, accompanied by the warm shortbread.