Press one of the prepared pie crusts into the bottom of a 9-inch pie pan. Make sure the crust goes up the sides as well. It's ok if excess overhangs the edges. Set the pan aside.
In a medium-sized bowl, whisk together the sugar, flour, and cinnamon until combined. Sprinkle about ¼ cup into the bottom of the prepared pie pan.
Top the mixture with the chopped rhubarb. The rhubarb should be enough to fill the pan, so you may need more or less.
Sprinkle the remaining flour mixture over the rhubarb. Evenly dot the top with butter.
Add the second pie crust to the top. Trim off the excess, then seal the top and bottom crusts together. Flute the edges if desired. Cut 2 to 3 slits in the top to allow steam to escape.
Optional: Before baking, brush the top lightly with the whisked egg yolk. Sprinkle additional granulated sugar on top.
Bake the pie in a preheated oven for 15 minutes. Cover the edges with foil or a shield if they are starting to brown too quickly. Reduce the temperature to 350º F and continue baking for about 40 to 45 minutes or until the filling is bubbly and the crust is golden brown. Allow the pie to cool before cutting and serving.