For the crust

1. Mix dry ingredients. In the bowl of an electric mixer, whisk together all-purpose flour, cake flour, salt, and sugar.
2. Cut in butter. Add the butter and shortening, and mix on low until mixture resembles coarse crumbs with some roughly pea-size butter lumps.
3. Add water. Drizzle 4 tablespoons ice water evenly over the mixture and mix on low until incorporated. Squeeze a small handful of dough together. If it doesn’t hold together, add more ice water, ½ tablespoon at a time, mixing until just incorporated, then test again.
4. Gather together. Gather dough together into a ball, then flatten into a ½-inch thick disk. Wrap dough in plastic and chill, until firm, about 1 hour.
5. Roll out. Have on hand a 9-inch pie pan that is at least 1 1/2-inches deep. Roll the chilled dough to 1/4-inch thick. Place the dough into the pie pan (see Notes). Gently press the dough into the bottom and side of the pan. Tuck under any excess dough around the top to reinforce the sides. Crimp the edges as desired.
6. Chill. Chill the dough in the pie pan until firm, about 15 minutes.
7. Prep oven. Heat oven to 375 degrees F.
8. Blind bake. Line the crust with parchment or foil and fill with pie weights. Place the pie pan on a baking sheet and bake until crust just barely starts to brown, 10-15 minutes. Remove baking sheet and pie pan from the oven and remove the lining and weights. Let cool slightly while making the lemon filling.

For the lemon filling

1. Mix. In a medium bowl, whisk together the melted butter, lemon juice, sugar, salt, and cornstarch. Whisk in the eggs, one at a time, until evenly mixed.
2. Pour. Pour lemon mixture into the warm pie crust.
3. Bake. Return the pie pan (still on a baking sheet) to the oven and bake on the bottom rack of the oven for 20-25 minutes, until the filling is almost set. Remove from the oven and place on a rack to cool for 15-20 minutes while preparing the streusel topping and blueberry filling.

For the streusel topping

1. Mix dry ingredients. In a small bowl, whisk together the flour, sugar, salt, and cinnamon.
2. Add butter. Mix the melted butter and vanilla together and then pour over the flour mixture. Stir with a fork until the mixture is evenly mixed and crumbly. Set aside.

For the blueberry filling

1. Mix. Place the blueberries in a 2-quart saucepan (if using fresh berries, rinse but don’t dry the berries first). Sprinkle the sugar, salt, and cornstarch over the berries and stir together until berries are evenly coated.
2. Cook. Cook the blueberries over medium-low heat, stirring frequently, until the berries start to release their juices. Bring mixture to a boil, stirring constantly, until the mixture thickens.
3. Fill. Remove from the heat and gently spoon the blueberry filling over the lemon layer.
4. Sprinkle. Sprinkle the streusel topping over the blueberry layer.
5. Bake. Return the pie to the oven and bake on the center rack until the crumbs are lightly browned,  15 to 20 minutes.
6. Cool. Remove pie from the oven and let cool to room temperature, then chill thoroughly before slicing and serving.