Add the softened butter and sugar to a large bowl. Beat them together until the mixture looks creamy and lightly fluffy. This takes about two minutes, and it builds the smooth base of the filling.
Add the flour, salt, and eggs to the bowl. Mix until the batter is smooth and no lumps remain. Scrape down the sides once so nothing hides at the bottom.
Pour in the buttermilk, lemon juice, and vanilla. Stir gently until everything is combined. Do not panic if it looks curdled, because that is completely normal and it bakes up smooth.
Set your unbaked crust on a cookie sheet. Pour the filling in slowly and stop just below the rim. The filling puffs as it bakes, so the sheet will catch any spills.
Slide the pie into a 350°F oven and bake for 50 to 55 minutes. The top should be lightly golden and the edges mostly set. The center should still jiggle a little, since it firms up as it cools.
Move the pie to a cooling rack and let it rest for about 30 minutes. Cover it loosely and refrigerate for 2 to 3 hours. This chilling time is what gives you neat, clean slices.
Score the top with a large serrated knife first. Then cut through with a paring knife and wipe the blade between slices. Lift each piece with a pie server and serve it cold, warm, or at room temperature.