Mash about 2 C. raspberries to measure 1 C. Place in saucepan. Add sugar, cornstarch, and water. Bring to a boil, stirring constantly. Cool and stir 2 minutes longer. Strain to remove berry seeds if desired. Cool to room temperature, about 20 minutes.

Meanwhile, for filling, beat cream cheese, whipped topping, and confectioners sugar in a mixing bowl. Top with remaining raspberries. Pour cooled raspberry sauce over top. Refrigerate until set, about 3 hours.