• If Making Homemade Pie Crust: Mix the flour and salt together in a large bowl. Add the chilled butter and shortening. Using a pastry cutter or two forks, cut the butter and shortening into the flour until it resembles coarse crumbs.
• Drizzle ice water into the crumbs, 1 tablespoon at a time, until large clumps begin to form while stirring with a rubber spatula. Most often, I add 1/2 cup of ice water.
• Transfer the dough to a floured surface. Bring the dough together with floured hands and shape into a ball, kneading gently as necessary until the flour is fully incorporated. Divide dough in half and flatten into 1-inch thick discs. Wrap each in plastic wrap and refrigerate for at least 2 hours.
  
  

For the Filling

• Stir the rhubarb, brown sugar, granulated sugar, cornstarch, salt, lemon juice, lemon zest, and vanilla extract together in a large bowl. Set aside.
• Preheat the oven to 400°F and line a baking sheet with foil for easy cleanup.

Assembling the Pie

• On a floured surface, roll out in one chilled dough disc. Rolling in one direction, rotate the dough a quarter turn each time until you have a 12-inch circle. Carefully fit the dough into a 9-inch pie plate.
• Spoon the rhubarb filling into the dough and smooth with a spatula. Place butter pieces on top of the filling.
• Making the lattice: Remove the second pie dough disc from the refrigerator. As before, roll the dough into a 12-inch circle. Using a sharp knife, pizza cutter, or pastry wheel, cut strips about 1 inch wide. Place half of the strips over the pie filling, working every other strip from the shorter side across. Then carefully weave the strips crosswise over and under the first strips laid on the pie. Press the edges of the strips into the bottom pie crust edges to seal.
• Fold the overhanging dough upward into a roll and seal. Crimp the edges of the pie dough.
• Lightly brush the top of the pie crust with beaten egg white and sprinkle with sugar if desired.
• Place the pie onto the foiled lined sheet and bake in preheated 400°F oven for 20 minutes. Reduce the heat to 350°F and bake for an additional 25-30 minutes. About halfway through, I often place a pie shield over the crust to prevent over-browning. Be sure to bake until the filling is bubbling.
• Let the pie cool completely at room temperature before serving. This could take 2 to 3 hours. Bringing it to room temperature lets the filling thicken.