Wash and zest the lemons carefully, then squeeze fresh lemon juice and set both aside before starting the cream mixture. Preparing the citrus in advance keeps the cooking process smooth and prevents overcooking later.
Combine heavy cream and sugar in a saucepan over medium heat, stirring until the sugar fully dissolves. Allow the mixture to simmer gently for several minutes so the cream slightly thickens and develops a rich texture.
Remove the saucepan from the heat and stir in the fresh lemon juice, zest, and vanilla extract. The citrus naturally thickens the cream, creating the smooth consistency lemon posset is known for.
Strain the mixture through a fine mesh sieve into serving glasses or ramekins to ensure the final dessert stays silky and free from zest clumps.
Refrigerate the filled dishes for at least 4 hours or overnight until fully set. Chilling gives the dessert its creamy spoonable texture and enhances the citrus flavor.
Garnish with berries, lemon zest, mint, or crushed cookies immediately before serving so the toppings remain fresh and crisp.
