Preheat oven to 425 degrees F.   Place pie crust in pie pan and flute edges as desired.

In a medium bowl, stir together 3/4 cup sugar, 2 tablespoons of flour, and salt. Mix in the sour cream, egg and vanilla until smooth. Add apples, and stir to coat. Scrape the mixture into the pie shell.

Bake for 15 minutes in the preheated oven, then reduce heat to 350 degrees F and continue baking for 25 minutes more.

While the pie is baking, prepare the topping in a medium bowl. Stir together 2/3 cup of flour, 2/3 cup sugar, and cinnamon. Cut in the butter until the mixture resembles fine crumbs.

After the 25 minute bake time has passed, cover the top of the pie with the crumb topping, and continue to bake for 10-15 minutes, or until topping is lightly browned and apples are tender. Allow the pie to cool, then refrigerate until chilled before serving.