Rinse, air dry, and pick through berries removing any bad ones. Place 3/4 of the berries into a large, deep saucepan, add sugar and fold. Heat over medium-high heat, and fold berries, scraping bottom and sides of pan until sugar has dissolved and there is juice. Add cornstarch slurry and cook and stir (gently) over medium heat until sauce is thickened and clear. Remove from heat and gently fold in rest of ingredients until butter has melted. Set aside until completely cooled down. Chill if not making pie right away. (*Can refrigerate overnight or up to 2 days.)
Preheat oven fully to 400º F. Spoon filling into pie pastry-lined 9-inch deep dish pie plate. *Any extra filling can be served over ice cream later. Top with second pie crust, seal and flute edges, and poke top with a knife several times to vent. Or you can make a lattice top or top of your own choice.
Bake in preheated oven for about 35 - 45 minutes, or until crust is done and browned, and filling is just starting to bubble. (It's always a good idea to place a sheet of foil under neath on rack below to catch any drips that may occur.) ALL ovens bake differently, so check pie for doneness after 35 minutes, then bake as long as needed.
Let cool completely before slicing. Serve with ice cream or whipped cream if desired.