Preheat oven to 350°F. Place pie crust in a 9-inch pie pan and pinch edges. Do not pre-bake. In a small bowl mix the Topping cinnamon and sugar, set aside.
Beat the butter and sugar in a large bowl until light and fluffy. Add the flour, salt and eggs. Beat again until combined and pour in the buttermilk, lemon juice and vanilla. Stir to incorporate, mixture will look curdled.
Pour filling into pie crust and bake on center shelf for 55 minutes and top is puffed and golden. Open the oven door and turn off heat. The pie filling should deflate evenly and may be slightly jiggly. It will set as it cools.
Remove from oven after 5 minutes and sprinkle the cinnamon sugar over the top. let cool completely. Serve with whipped cream, optional.