Preheat oven to 350 degrees F.

In a medium bowl, combine the graham cracker crumbs, sugar, and melted butter. Mix with a fork until combined, then set aside ¼ cup of the crumb mixture for later.

Press the remaining crumb mixture into a 9x13-inch baking dish. Bake for 8-10 minutes until set.

In a large saucepan, combine the rhubarb, sugar, and cornstarch and toss to combine. Add the water, then cook, stirring frequently over medium heat, until the rhubarb softens and the filling thickens, about 5-7 minutes. Remove from heat and stir in the package of strawberry jello.

Pour over the graham cracker crust then sprinkle with the marshmallows. Set in the fridge to chill completely, about 1 hour.

In a large bowl, whisk the cold milk and vanilla pudding mix together for 2 minutes until it starts to set. Set aside.

Prepare the whipped cream by beating in a large bowl with the powdered sugar using a hand mixer until soft peaks start to form. Fold the vanilla pudding into the whipped cream, then spread over the chilled rhubarb layer.

Sprinkle with the reserved graham cracker crumb mixture.

Chill for at least 2 hours to firm up, then slice and serve.