In a medium saucepan over medium heat, add the heavy cream, white chocolate, vanilla and sugar. Stir frequently and bring to a gentle boil. Let boil for 5 minutes, stirring constantly. Remove from heat and whisk in the lemon juice and zest until combined.
Let the mixture set for 10 minutes before pouring into the lemon shells. Transfer the posset to the fridge and let it set up, uncovered for at least 3 hours. Garnish with lemon zest and white chocolate and enjoy!
Store posset in the fridge, covered for up to 5 days. Best to enjoy within 3 days.