To a small saucepan, add Heavy Cream (300 milliliter) and Caster Sugar (1/3 cup). Over a low heat, stir together until the sugar is dissolved.
Increase temperature and bring the mixture to a boil. Once boiling, turn the temperature down to low and let simmer for three minutes, stirring constantly.
Remove pot from heat and gradually stir in Lemon (1/3 cup). Let cool for five minutes.
Divide Posset between serving glasses, cover glasses with plastic wrap, and chill in the fridge for about 4 hours.
Serve with a bit of Lemon (to taste) on top and enjoy!