Beat butter and sugar in a large mixing bowl till creamy, about 2 minutes
Add flour, salt, and eggs. Beat till smooth. Add buttermilk, lemon juice, and vanilla and blend well. (Mixture may appear curdled if you use cool butter or eggs.)
Pour filling into a 9" pie crust. I like to use a deep dish.
Place pie pan on a cookie sheet and bake for 30 minutes.
Walnut Streusel
Using a medium bowl and fingers, pinch together all streusel ingredients to create a crumble.
Remove pie from oven and top the walnut streusel on top.
Place back into the oven and continue to bake for 20 minutes.
Allow to cool on a cooling rack for 30 minutes before serving.