For the pie crusts: Whisk together flour, salt and sugar in a large bowl. Cube butter and add to flour mixture. Use a pastry blender to cut butter into dry ingredients until pea-sized pieces of butter are scattered throughout. Add ice cold water 1 tablespoon at a time until dough starts to form a ball. Turn the dough out onto a floured work surface and knead briefly until dough is formed. Divide dough in half and shape into 2 flat disks. Cover with plastic wrap and refrigerate at least 1 hour.
Roll out one dough disk into a 12-inch circle. Press the dough evenly into bottom and up sides of 9-inch pie plate. Refrigerate while you prepare the rest of the recipe.
Place rhubarb in a large colander and pour boiling water over the slices. Shake colander to remove excess water and set aside.
Preheat oven to 400°F.
Whisk together sugar, flour and tapioca flour in a large bowl. Add rhubarb and toss to coat using a large spoon. Let mixture stand 10 minutes. Add egg and stir together. Pour mixture into prepared pie crust in pie plate. Dot filling with pieces of butter.
Roll out second dough disk into a 12-inch circle. Cover pie filling with rolled dough and edges using a fork. Cut slits in top of crust so steam can escape during baking.
Bake pie 15 minutes. Lower oven temperature to 350°F and bake an additional 40 minutes, or until the crust is golden and the filling is bubbling.
Let cool slightly before slicing. Serve warm with whipped cream, if desired.