1. Pastry: Combine flour, sugar, baking powder, and salt in the bowl of a food processor. Add butter and pulse until butter is incorporated, leaving pieces no larger than the size of a lentil. Transfer mixture to the bowl of a stand mixer fitted with the paddle attachment. With machine running on low speed, slowly add 3/4 cup buttermilk. Process until dough just comes together but does not form a ball, adding up to 1/4 cup more buttermilk if needed to bring dough together. Finish by hand, firmly pressing dough together to incorporate wet into dry. When there are still some dry crumbles, divide dough in half. Transfer each half to a piece of plastic wrap. Using the plastic wrap, form each into a disk and wrap well. Chill until firm. Divide each disk into 6 equal pieces. Working with one piece at a time, roll out on a lightly floured work surface into a 7-inch circle. Transfer to a baking sheet and place in freezer until well chilled. Repeat process with remaining dough.

2. Filling: Toss apples to coat with lemon juice in a large bowl. Add granulated sugar, 1/2 cup brown sugar, flour, cinnamon, and salt; toss to combine. Gently stir in vanilla and heavy cream. Melt butter in a large pot over medium heat, and add remaining 1/2 cup brown sugar and apple mixture. Cook, stirring occasionally, until mixture thickens. Let cool and transfer to refrigerator until chilled.

3. Streusel: Combine flour, sugar, and butter in a medium bowl. Mix until coarse crumbs form; add pecans and mix to incorporate. Transfer to refrigerator until chilled.

4. Finishing: Preheat oven to 375 degrees. Fit one dough round into a 5-inch metal pie tin to create a bottom crust. Fill with about 1/2 cup of the apple filling mixture. Sprinkle with 1 tablespoon streusel mixture. Gently brush edges of dough with egg white to moisten. Top with a second round. Gently press to adhere. Trim edges flush to end of pan and crimp, as desired. Brush top with egg white. In a small bowl, combine cinnamon and sugar and sprinkle over pie. Place on a rimmed baking sheet and transfer to refrigerator until chilled. Repeat process with remaining ingredients.

5. Transfer pies to oven and bake until golden brown, 45 to 50 minutes. Let cool on a wire rack. Serve warm or at room temperature.