In a small saucepan, heat the cream and sugar over medium heat. Stir continuously until the sugar dissolves, then let it simmer vigorously for 8-12 minutes.
Take the saucepan off the heat and mix in the lime juice, vanilla, and lime zest (optional).
Allow the mixture to steep for 5 minutes.
Transfer the mixture into a 10-ounce ramekin or a similarly sized dish. Cover and refrigerate for at least 3-4 hours to chill.
Notes
Chilling Time: Chill the posset for at least 4 hours, or overnight for the thickest consistency.
A GreatMake-Ahead Dessert: You can make this custard ahead of time. In fact, it should be refrigerated for 3 hours to set before eating. The custard will keep well for 2 days in the refrigerator.
Important Note: Although a posset is simple to make with just a few ingredients, it's crucial to adhere to the instructions precisely. Pay special attention when simmering the cream and sugar mixture. Aim for a vigorous simmer, characterized by noticeable movement in the cream, wisps of steam, and numerous small bubbles. Avoid letting the mixture reach a rolling boil, which is indicated by larger bubbles and more intense activity in the pot. By maintaining the simmer as described, you'll achieve the perfect consistency for your posset.