In a small saucepan, combine the cream and sugar. Set it over medium heat. Gently bring the mixture to a robust simmer, stirring continuously until the sugar is fully dissolved. Let it simmer for 8-12 minutes.
Take the saucepan off the heat. Mix in the grapefruit juice, vanilla, and grapefruit zest. Allow this mixture to sit for 5 minutes to infuse the flavors.
Let the mixture steep for 5 minutes.
Transfer the mixture into a 10-ounce ramekin or a similar dessert dish. Cover it and refrigerate for a minimum of 3-4 hours to set.
Notes
Careful Simmering: While preparing a posset, ensure the cream and sugar mixture simmers vigorously but doesn't boil. Look for consistent steam and small bubbles, avoiding a rolling boil with large bubbles.
Advance Preparation:
Set Time: Possets need at least 3 hours in the fridge to set, making advance preparation ideal.
Shelf Life: Once prepared, your posset can stay fresh in the refrigerator for up to 2 days.