Preheat your oven to 425°F (220°C). In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, and salt until well combined. Cut in the softened unsalted butter until the mixture resembles coarse crumbs. Gradually add ice water, one tablespoon at a time, until the dough comes together. Roll out the dough and transfer it to a pie pan, pressing it into the edges.

In a medium bowl, combine the chopped rhubarb, sugar, cornstarch, vanilla extract, and cinnamon if using. Stir gently until the rhubarb is well coated. Pour this filling into your prepared pie crust, ensuring it's evenly spread.

In a separate bowl, mix the brown sugar, rolled oats, flour, and melted butter until crumbly. Evenly sprinkle the topping over the rhubarb filling.

Place the pie in the preheated oven and bake for 15 minutes at 425°F (220°C). Reduce the temperature to 350°F (175°C) and continue baking for an additional 30-35 minutes until bubbling and golden brown.

Once done, remove the pie from the oven and let it cool for at least 15-20 minutes before serving.