Turn off the heat.
Add the vanilla extract.
Then gradually stir in the fresh lemon juice.
The cream will begin thickening as the acid reacts with it.
It may also develop a very pale yellow color.
Step 5 – Strain
For an especially smooth posset, pour the mixture through a fine-mesh sieve.
This removes the zest and any tiny bits that may have formed.
If you enjoy visible zest, you can skip this step.
Step 6 – Cool Slightly
Let the mixture rest for about 10 minutes.
This prevents the very hot cream from softening or weakening the lemon shells.
Step 7 – Fill the Lemon Halves
Carefully spoon or pour the posset mixture into the prepared lemon shells.
Fill them close to the top without overflowing.
Step 8 – Chill Until Set
Transfer to the refrigerator.
Chill for at least 2–4 hours, or preferably overnight.
The center should feel set and creamy rather than liquid.
If it still jiggles heavily, give it more time.
The Secret to Getting Lemon Posset to Set
Lemon posset works because of a simple chemical reaction.
Heating the cream concentrates it slightly and changes the milk proteins.
When fresh lemon juice is added, the acidity causes those proteins to tighten and create a thick, creamy structure.
The most important factors are using enough heavy cream, enough lemon juice, and allowing sufficient chilling time.
Do not boil the cream aggressively. Very high heat can destabilize the mixture and increase the chance of separation once the lemon juice is added.
How to Brûlée the Sugar
Wait until just before serving.
Sprinkle a thin, even layer of granulated sugar over the surface of each fully chilled posset.
Use a kitchen torch to melt and caramelize the sugar.
Keep the torch moving continuously so one area does not burn.
The sugar should turn deep golden and glossy.
Let it sit for about 30–60 seconds until the surface hardens.
Serve immediately.