In a medium saucepan, combine sliced rhubarb, strawberries, granulated sugar, and butter over medium heat. Cook for 8-10 minutes, stirring occasionally until the fruit is tender and juicy. Gradually whisk in corn starch, cooking for an additional 2-3 minutes until the mixture thickens. Remove from heat and let cool completely.

Preheat your oven to 350°F (175°C). In a mixing bowl, combine crushed graham crackers and melted butter until the mixture resembles wet sand. Press this mixture firmly into the bottom of a 9x13-inch dish. Bake for 10 minutes until lightly golden. Remove from the oven and let cool completely.

In a separate bowl, whisk together instant vanilla pudding mix with milk as directed on the package. Mix until thick and creamy. Set aside while the crust cools.

Gently fold the thawed Cool Whip and mini marshmallows into the thickened pudding mixture until well incorporated. Evenly spread the fruit mixture over the cooled graham cracker crust. Pour the creamy pudding mixture over the fruit layer, spreading it carefully for an even distribution.

Cover the dish with plastic wrap or aluminum foil and refrigerate the assembled dessert for at least 1 hour, allowing the flavors to meld and the pudding to set.

After chilling, slice the dessert into squares and plate them for your guests. Garnish with extra sliced strawberries or crushed graham crackers if desired.