In a mixing bowl, combine flour, sugar, and salt. Cut in cold, cubed butter until mixture resembles coarse crumbs, about 3-5 minutes.

Gradually add ice cold water, mixing gently until the dough forms. Be careful not to overwork it.

Shape dough into a disc, wrap in plastic, and refrigerate for at least 1 hour.

In a large bowl, mix together sugar, corn starch, cinnamon, and vanilla extract. Toss in sliced rhubarb, allowing to rest for 15 minutes.

Preheat oven to 375°F.

Remove dough, divide into 6 portions, roll each into a 1/4 inch thick circle.

Place dough circles on a baking sheet. Spoon rhubarb filling into the center, leaving a 1-2 inch border. Fold edges over the filling.

Bake for 45-60 minutes until the crust is golden and filling bubbles.

Allow to cool on the baking sheet for at least 10 minutes before serving.