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Prepare crust - preheat oven to 350°F (175°C). In a large bowl, mix graham cracker crumbs, sugar, and melted butter. Reserve 2 tablespoons of mixture, and press the rest into a 9x13-inch baking dish.
Bake crust - bake for 8-10 minutes until lightly golden. Remove from oven and set aside to cool.
Cook rhubarb - in a large saucepan, combine chopped rhubarb and water. Cook over medium-high heat until softened, 3-4 minutes. Stir in sugar, cornstarch, and strawberry jello until thickened and clear.
Assemble first layers - pour hot rhubarb mixture over cooled crust and top evenly with mini marshmallows. Refrigerate until completely set, about 2 hours.
Make pudding layer - in a large bowl, whisk together instant vanilla pudding mix and milk until thick. Gently fold in thawed whipped topping until smooth.
Finish bars - spread pudding mixture over set marshmallow layer. Sprinkle with reserved graham cracker crumbs. Refrigerate until completely set before slicing and serving.