Crush graham crackers into fine crumbs and mix with melted butter. Press into a 9x13 inch dish and chill.

Combine rhubarb, sugar, cornstarch, and water in a saucepan. Cook over medium heat until thickened, about 20-30 minutes.

Whip heavy cream until stiff peaks form, then fold in mini marshmallows.

Spread cooked rhubarb filling over the chilled crust.

Spread whipped cream mixture over the rhubarb filling.

Prepare pudding mix as per package instructions and spread over the whipped cream layer.

Refrigerate for at least 6 hours, sprinkle with graham cracker crumbs before serving.