Preheat your oven to 425°F (220°C).
In a mixing bowl, combine the chopped rhubarb, granulated sugar, all-purpose flour, salt, and vanilla extract. If using cinnamon, add it to the mixture as well. Stir well to coat the rhubarb evenly.
Roll out the pie crust and fit it into the 9-inch pie pan, leaving a little overhang to fold under and crimp the edges.
Pour the rhubarb mixture into the pie crust, spreading it evenly. Dot the top with small pieces of butter.
Cover the pie with the second rolling of pie crust, or create a lattice crust for a decorative touch. Trim any excess dough, fold the edges under, and crimp to seal.
Brush the top crust with the beaten egg to create a beautiful golden finish.
Cut a few slits in the top crust to allow steam to escape.
Bake the pie in the preheated oven for 15 minutes, then reduce the temperature to 350°F (175°C) and continue baking for an additional 30 minutes, or until the filling is bubbly and the crust is golden brown.
Once done, remove the pie from the oven and let it cool for at least 2 hours before serving to allow the filling to set.