In a large mixing bowl, beat the softened unsalted butter and granulated sugar together. You want to mix until the combination is nice and creamy. This step is vital; it incorporates air into the mixture, giving your pie a light texture. Don't rush this! You’ll know it's ready when it looks pale and fluffy.

Next, add in the all-purpose flour and salt. Blend this mixture into your butter and sugar until well combined. You want to ensure there are no lumps—smooth is the name of the game.

Now, crack in the three large eggs one at a time, beating after each addition. This not only helps incorporate them into the mixture but also adds richness to your filling. Keep mixing until everything is uniform and smooth, creating a lovely creaminess that will bake beautifully.

Pour in the buttermilk (or yogurt if you’re feeling adventurous), lemon juice, and vanilla extract. Mix this all together. Don’t fret if the mixture looks a bit curdled; that’s perfectly normal. This eccentric look contributes to the dreamy texture of your pie.

Grab your 9-inch deep-dish pastry crust. Carefully pour the filling into the crust. The mixture may appear thin, but it will set up nicely in the oven.

To prevent any spills, place your pie pan on a baking sheet and slide it into an oven preheated to 350°F (175°C). Bake for about 50-55 minutes. You want the top to be lightly browned and the pie to be mostly set — a gentle jiggle in the center is a good sign of its creaminess.

Once baked, transfer the pie to a cooling rack. Allow it to cool completely before slicing. This pie is delightful served at room temperature, but if you have leftovers, you might want to chill them in the refrigerator.