Combine sugar, cornstarch and salt in saucepan.

Add water and 2 cups blueberries. Cook over medium heat, stirring constantly until mixture comes to a boil, and is thick and clear. Mixture should be quite thick.

Remove from heat and stir in butter and lemon juice.

Cool, place remainder of blueberries in pie shell. Top with cooked berry mixture.

Chill and serve garnished with whipped cream.