Mix together cornstarch and 1/2 cup sugar. Bring to boil 2/3 of the blackberries with the cornstarch/sugar mixture, lemon zest, and the lemon juice. Boil until thickened. Pour into the prebaked pie shell. While mixture is still warm, layer remaining blackberries on top. Chill for a minimum of 4 hours.
Right before serving, whip cream with vanilla and remaining sugar until you have reached "stiff peaks". Pipe, or simply dollop, whipped cream onto pie. Garnish with a few fresh berries.