Lemon Posset

Combine the double cream, caster sugar, and lemon zest in a heavy saucepan. Stir over medium heat until the sugar dissolves and the cream begins to simmer. Raise the heat slightly and bring to a full, rolling boil — the cream will bubble up dramatically, so use a pan at least three times the volume of the cream. Boil for exactly 3 minutes, stirring and watching carefully. The rolling boil reduces the cream slightly and concentrates the fat content, which is essential for the setting mechanism. Time this carefully — under 3 minutes and it may not set; over 4-5 minutes and it may taste scorched.

Remove the boiled cream from the heat and allow to cool for 2-3 minutes — it should still be hot but no longer violently boiling. Strain in the fresh lemon juice, stirring as you go. The juice must be freshly squeezed and strained of all pulp and seeds. As the acid meets the hot cream, the mixture will thicken perceptibly within seconds — the protein chains in the cream contract around the fat droplets in the presence of acid, setting the posset without any gelatin. Stir briefly until combined, then pour immediately into serving glasses or ramekins through a fine sieve.

Pour the hot posset mixture through a fine-mesh sieve into individual serving glasses or shallow ramekins, filling each to about 1 cm below the rim. Allow to cool at room temperature for 20 minutes, then refrigerate uncovered for at least 3 hours, or overnight. The posset will set from a thick liquid into a firm, trembling cream — fully set but with a delicate wobble. Do not cover while cooling or condensation will drip in and prevent a clean surface.

Just before serving, run a fine-mesh zester over a fresh lemon and scatter a pinch of zest over the white surface of each posset. Press a shortbread biscuit or langue de chat into the edge of each glass at an angle so it rests against the rim like a sail. Serve immediately from the refrigerator — the posset should arrive at the table cold, the contrast between the cold, set cream and the crumbling warm shortbread being the final textural pleasure.