Place 2 cups fresh blueberries in baked pie shell.
Combine sugar and cornstarch in saucepan. Add water and 2 cups blueberries. Cover and heat until mixture comes to a boil. Continue cooking over low heat for 2 minutes, stirring constantly, until mixture has thickened and become clear. Remove from heat and stir in butter. Cool.
Pour cooled mixture over berries and chill at least 2 hours. Serve with sweetened whipped cream.