Preheat oven to 375 degrees F, adjust oven rack to the center of the oven.
In a large bowl, toss together the diced rhubarb with the sugar, cornstarch and vanilla extract until rhubarb is fully coated. Transfer the rhubarb mixture into an 8x8 baking dish, set aside.
In a medium bowl, stir together flour, baking powder, sugars, and salt until combined. Add melted butter and stir until fully combined. Mixture should resemble wet sand.
Sprinkle crumble topping over the rhubarb evenly, making sure topping reaches the edges of the pan.
Bake rhubarb crumble at 375 degrees for 35-40 minutes, until filling is bubbling and topping is starting to turn golden brown.
Allow to cool before serving. Serve with vanilla ice cream or whipped cream if desired.