In a large bowl, combine flour and salt. Add butter and use a pastry blender to cut butter into the flour until it resembles coarse crumbs.
Add water to the flour and butter mixture 1 tablespoon at a time, using a fork or your hands to mix until the dough comes together. You should still see small bits of butter, which helps with the flakiness.
Using your hands, shape the dough into a disc. Wrap in plastic wrap and refrigerate for at least 4 hours or overnight.
On a lightly floured surface, roll out the dough into an 11-inch circle. Gently fold in half and then unfold over a 9-inch pie dish. Press dough into the dish, trim excess, and flute the edges.
Place chilled dough on a generously floured surface and roll out to an 11-inch circle, adding more flour to your rolling pin as needed.
Carefully roll the dough onto the rolling pin, then unroll over a 9-inch pie dish. Press dough evenly into the bottom and sides of the dish. Trim any excess dough and flute the edges.
Preheat oven to 350°F.
Filling: Add the apples, butter, and sugar to a large saucepan over medium-high heat. Cook, stirring constantly for 5-7 minutes or until half the apples have dissolved. Transfer the mixture to a bowl and let it cool.
Custard: Add the flour and sugar to a large bowl and whisk together. Add the eggs, then vanilla, then sour cream, whisking after each addition.
Topping: Add the flour, sugar, baking powder, and cinnamon to a medium bowl and mix. Add the melted butter and mix until a crumbly mixture forms.
Add the cooked apples to the custard filling and mix to combine. Pour this over the pie crust in the pie pan. Sprinkle the crumbs over the top. Bake for 50-60 minutes, or until the crumbs are a deep golden color.
Let the pie cool on a wire rack. Cool completely before cutting and serving.