Bring cream, sugar, and lemon zest to a boil in a medium saucepan
Stir and continue to boil until the mixture is reduced to 2 cups, approximately 5-10 minutes
Remove the pan from heat and stir in the lemon juice
Divide the crushed amaretti cookies between individual glasses
Pour the lemon mixture over the cookies carefully to just cover them
Refrigerate until set, approximately 30 minutes
Pour the remaining lemon mixture into the glasses and refrigerate for at least 4 hours or overnight
Heat the blueberries with water until boiling
Mix cornstarch with a little water and stir into the blueberries
Boil until the liquid thickens, then set aside and cool
Spoon the blueberry topping onto the possets before serving
