In a medium saucepan, combine heavy cream, sugar, and lemon zest.

Heat over medium heat, stirring occasionally, until the sugar is fully dissolved and the cream begins to simmer gently. Do not let it boil.

Remove the saucepan from heat and stir in the fresh lemon juice. The mixture will thicken slightly as the acid interacts with the cream.

Add vanilla extract if desired and stir to combine.

Pour the posset mixture into individual serving glasses or ramekins.

Allow the posset to cool to room temperature, then refrigerate for at least 2–3 hours, or until set.

Just before serving, top each posset with fresh berries and garnish with mint leaves if desired.