Preheat your oven to 400 degrees. In a large bowl, whisk the three eggs until they are light and foamy. Add the sugar and flour, and whisk until you get a smooth mixture.
Add the buttermilk, melted butter, vanilla, lemon zest and lemon juice and whisk to combine. We should note that if you want a sharper, richer flavor in your buttermilk pie, you can swap two tablespoons of bourbon or rum in for the vanilla extract.
It really takes just a few minutes total time to whip up the pie filling; once it's ready, pour it into a pie shell. Homemade or store bought is fine, though you'll want to make sure that you don't overfill the pie pan.
Pop your buttermilk pie in the oven at 400 degrees for 10 minutes to start, then lower the oven temperature to 350 degrees and bake the pie for another 30 to 40 minutes. Watch the pie carefully for the last 10 minutes or so; you want the pie to be golden brown and for the pie filling to set, which means it may be a bit jiggly in the middle, but not liquid. Watch the crust, too; if it gets too brown, cover it with foil for until the pie has finished baking.
Let the buttermilk pie sit until it cools to room temperature. Serve with whipped cream if you like and enjoy your new favorite pie!