To make crust, combine flour, ¼c. sugar and salt in bowl. Cut in chilled butter until mixture resembles coarse crumbs. Pat in bottom of ungreased 13 X 9 X 2" pan. Bake in moderate oven (350°) 15 to 18 minutes, until lightly browned. Cool.
For filling, combine cornstarch and 1 c. sugar in saucepan. Add raspberries and cook, stirring constantly, until mixture comes to a boil and is clear. Cool slightly. Pour over crust. Chill.
To make topping, place marshmallows and milk in a saucepan; cook over low heat, stirring frequently, until marshmallows are melted. Cool. Fold whipped cream into marshmallow mixture; spread over chilled raspberry filling. Chill.