Preheat the oven to 425°F (220°C).
In a mixing bowl, combine the flour and cold butter. Mix until the mixture resembles coarse crumbs.
Add ice water a tablespoon at a time until the dough comes together. Divide the dough in half, wrap in plastic wrap, and refrigerate for at least 30 minutes.
Roll out one half of the dough and fit it into a 9-inch pie dish. Trim the excess dough.
In another bowl, mix the chopped rhubarb, sugar, and flour. Pour the mixture into the pie crust and dot with butter.
Roll out the second half of the dough and place it over the filling. Trim, seal, and flute the edges. Cut slits in the top crust to allow steam to escape.
Bake in the preheated oven for 45 minutes or until the crust is golden brown and the filling is bubbly.
Let the pie cool before serving.