Wash and zest the lemons. Wash and dry the 3 large lemons thoroughly. Before cutting them, finely grate 1 tablespoon of zest from the yellow outer peel, avoiding the white pith. Set the zest aside.
Prepare the shells. Cut each lemon in half lengthwise. Carefully loosen and remove the flesh with a small spoon without cutting through the peel. Place the six empty shells in a muffin pan or small baking dish to keep them level.
Measure the juice. Transfer the lemon flesh to a bowl and press it with a citrus reamer or wooden muddler. Strain through a fine-mesh strainer, then measure ⅓ cup (80 ml) of fresh juice and set it aside.
Heat the cream mixture. Add the 2 cups (480 ml) heavy cream, ⅔ cup (135 g) sugar, prepared lemon zest and 1 teaspoon vanilla to a medium saucepan. Heat over medium-low heat, stirring until the sugar dissolves completely.
Simmer gently. Bring the cream to a gentle simmer and cook for 3 to 4 minutes, stirring occasionally. Do not allow it to boil aggressively. Remove the saucepan from the heat.
Add the lemon juice. Slowly pour the measured lemon juice into the warm cream while stirring gently. Let the mixture rest for 5 minutes.
Strain, fill and chill. Strain the cream mixture through a fine-mesh strainer into a clean measuring jug, then divide it evenly between the prepared shells. Carefully refrigerate the filled shells. Chill uncovered until the surface is cool, then loosely cover and continue chilling for at least 4 hours, or until completely set. Overnight chilling also works.
Caramelize the topping. Just before serving, divide the reserved 2 tablespoons (25 g) sugar among the chilled possets in a thin, even layer. Use a kitchen torch to caramelize the sugar until golden brown and bubbling. Keep the torch moving to brown the sugar evenly without overheating the cream.
Finish and serve. Let the topping rest for 1 to 2 minutes until firm and crisp. Garnish with fresh mint leaves, if desired, and serve immediately.
