In a bowl, combine the cracker crumbs, sugar and butter. Set aside 1/4 cup for topping. Press the remaining crumb mixture onto the bottom of a greased 11x7-in. baking dish. Bake at 350° for 10-14 minutes or until lightly browned and set. Cool on a wire rack.
For filling, combine sugar and cornstarch in a saucepan. Gradually whisk in milk until smooth. Bring to a boil; cook and stir for 2 minutes or until thickened. Remove from the heat. Stir in a small amount of hot filling into egg; return all to the pan, stirring constantly. Bring to a gentle boil; cook and stir for 1 minute. Remove from the heat; stir in vanilla. Cool for 30 minutes, stirring several times.
Stir in pineapple. Pour over crust. Top with whipped cream (pan will be full). Sprinkle with marshmallows and reserved crumb mixture. Cover and refrigerate for at least 3 hours.