Begin by melting butter in a medium saucepan over low heat. In a mixing bowl, combine the melted butter with graham cracker crumbs until fully mixed and resembles wet sand. Press three-quarters of this mixture firmly into the bottom of a 9x13 inch pan to form a solid crust. Place the pan in the refrigerator to chill while you prepare the rhubarb filling.

In a large saucepan, combine chopped rhubarb, granulated sugar, water, and cornstarch. Bring the mixture to a boil over medium heat, constantly stirring until the rhubarb breaks down and the mixture thickens, which should take about 20-30 minutes. Once thickened to a pudding-like consistency, remove from heat and allow it to cool completely.

In a mixing bowl, whip the heavy cream using an electric mixer on medium speed until soft peaks form. Gradually add granulated sugar while continuing to beat until stiff peaks form. Gently fold in mini marshmallows.

Once your rhubarb filling is completely cool, spread it evenly over the chilled graham cracker crust. Then, spoon the whipped cream mixture over the rhubarb, smoothing it gently. Prepare instant vanilla pudding according to package instructions, and spread it uniformly over the whipped cream layer.

Sprinkle the remaining graham cracker mixture on top of the assembled layers. Cover the pan securely with plastic wrap and refrigerate for at least 6 hours or overnight to ensure the dessert sets beautifully.