Place cherries in medium saucepan and place over heat. Cover. After the cherries lose considerable juice, which may take a few minutes, remove from heat. In a small bowl, mix the sugar and cornstarch together. Pour this mixture into the hot cherries and mix well. Return the mixture to the stove and cook over low heat until thickened, stirring frequently. Remove from the heat and let cool. If the filling is too thick, add a little water, too thin, add a little more cornstarch.
Preheat the oven to 375 degrees F.
Use your favorite pie dough recipe. Prepare your crust. Divide in half. Roll out each piece large enough to fit into a 9″ pan. Pour cooled cherry mixture into the crust. Dot with butter. Moisten edge of bottom crust. Place top crust on and flute the edge of the pie. Make a slit in the middle of the crust for steam to escape. Sprinkle with sugar.
Bake for about 50 minutes. Remove from the oven and place on a rack to cool.