Preheat your oven to 375°F (190°C). Wash rhubarb thoroughly and trim away any leaves and tough ends. Cut into ½-inch pieces.
In a large bowl, toss the rhubarb with sugar, cornstarch, vanilla, and orange zest if using. Let this mixture sit for 10 minutes.
In a separate bowl, whisk together flour, both sugars, and salt. Add the cold diced butter and work it into the dry ingredients using your fingertips.
Transfer the rhubarb mixture to a 9×13-inch baking dish, spreading it evenly. Distribute the crumble topping over the fruit. Bake for 40-45 minutes until the topping is golden brown and the filling bubbles at the edges.
