Preheat your oven to 425°F (220°C).
In a large bowl, combine chopped rhubarb, sugar, cornstarch, vanilla extract, lemon juice, and cinnamon. Mix well.
Roll out one pie crust and place it into a pie dish.
Pour the rhubarb mixture into the pie crust, spreading it evenly.
Cover with the second pie crust, sealing the edges and cutting slits for steam to escape.
Brush the top crust with the beaten egg for a golden finish.
Bake for 15 minutes at 425°F, then reduce the heat to 350°F and bake for an additional 30 minutes.
Check for bubbling filling, indicating it's done.
Remove from the oven and let cool for at least 2 hours before slicing.
Serve and enjoy your homemade rhubarb pie!