Combine raspberries, ¼ cup water, and granulated sugar in a medium saucepan. Cook over medium heat, stirring with a wooden spoon to break up the berries, until bubbling and starting to thicken, about 8 minutes. Combine the remaining ¼ cup water and cornstarch in a small bowl. Add to the berry mixture and continue cooking, stirring, until it reaches jam-like thickness, about 3 minutes more. (You should have about 1 cup filling.) Remove from heat and set aside.