01 - Combine heavy cream and caster sugar in a medium saucepan. Heat gently over low heat, stirring until the sugar dissolves and the cream begins to simmer. Maintain a gentle boil for 2 to 3 minutes, stirring constantly. Remove from heat, then incorporate the lemon zest and juice. Allow to cool for 5 minutes, strain through a fine sieve into a measuring jug, and divide evenly into four serving glasses or ramekins. Refrigerate for a minimum of 3 hours until set.
02 - Preheat the oven to 320°F (160°C) fan or 356°F (180°C) conventional. Cream the softened butter with caster sugar until pale and fluffy. Add flour and salt, mixing until a soft dough forms. Lightly flour a surface and roll the dough to approximately 0.4 inches thick. Cut into fingers or rounds and place on a lined baking sheet. Prick each piece with a fork, sprinkle with caster sugar, and bake for 12 to 15 minutes or until pale golden. Cool on the baking tray for 5 minutes, then transfer to a wire rack to cool completely.
03 - Present the chilled lemon posset accompanied by two shortbread biscuits per serving.