2To make the filling, combine half the blueberries with the sugar, cornstarch, lemon juice, cinnamon and salt in a medium saucepan set over medium-high heat. Add 1/4 cup water if you're using fresh blueberries. Bring to a simmer and cook, stirring often, for a few minutes or until the berries begin to pop and the mixture thickens. Remove from the heat and stir in the remaining blueberries and the butter.
3Divide the pastry in half, making one piece slightly larger than the other. On a lightly floured surface, roll the larger piece out into a 12-inch circle; transfer to a pie plate and gently fit it in, without stretching. Pour in the filling. Roll out the remaining pastry, making a circle slightly larger than your pie plate. Cut rough holes for the eyes and nose, and make the mouth with the tip of a knife, without cutting all the way through. Transfer to the top of the pie and trim the excess crust around the edge of the pie plate, pinching the pastry together to seal and pressing down all the way around with the tines of a fork.