Combine flour, sugar and salt in a bowl. Cut in chilled butter until mixture resembles course crumbs.
Pat into the bap and bake at 350º for 15-18 minutes or until lightly brown.
Combine cornstarch and sugar in a saucepan. Cook, stirring constantly until it boils and is clear.
Cool slightly, mix in the raspberries and pour over the crust.
Place marshmallow and milk in a saucepan, cook over low heat, stir frequently until marshmallow melts.
Cool. Fold in whipped cream. Spread mixture over chilled raspberry mixture.
Chill for 1-2 hours before serving