Preheat your oven to 350°F (175°C). Gather your ingredients and prepare a clean workspace.

Combine 1 cup of melted butter, 1 cup of granulated sugar, and 2 cups of all-purpose flour in a medium bowl. Mix until the mixture resembles coarse crumbs. Set aside half for topping.

Press half of the crumb mixture into the bottom of an 8x8-inch baking pan to form an even layer.

Toss 2 cups of chopped fresh rhubarb with 1 cup of sugar. Spread the rhubarb mixture evenly over the crumb layer.

Sprinkle the reserved crumb mixture over the rhubarb layer, covering it completely.

Bake in the preheated oven for 35-40 minutes until the topping is golden brown and the filling is bubbling.

Allow the squares to cool in the pan for 30 minutes to 1 hour before slicing.