Simmer cream and sugar: In a small saucepan, combine 1 cup heavy cream and 1/3 cup sugar. Bring to a gentle simmer over medium heat, stirring occasionally, until the sugar is fully dissolved, about 2-3 minutes.
Remove from heat and flavor: Take the pan off the heat. Stir in 1/3 cup fresh lemon juice, the zest of one lemon, and 1/4 tsp vanilla extract.
Strain: Pour the mixture through a fine mesh sieve into a bowl or measuring cup to remove zest and any curdled bits. This gives your posset that silky-smooth texture.
Pour and chill: Spoon or pour the posset into ramekins, small glasses, or hollowed-out lemon halves. Place in the fridge or freezer and chill for at least 4 hours or overnight, until set.
Notes
(1) For best results, use full-fat heavy cream and fresh lemon juice.(2) To serve in lemon cups, scoop out halved lemons and freeze the shells before pouring in the posset.(3) Posset can be made up to 2 days in advance.(4) Add a brûlée topping or a shortbread cookie on the side to make it extra special.