Preheat oven to 350°F (175°C). Place the unbaked pie crust into a 9-inch pie dish and set aside.

In a large bowl, whisk sugar and flour together until combined.

Add the eggs one at a time, whisking well after each addition.

Pour in the buttermilk, melted butter, and vanilla (plus lemon zest if using). Mix until smooth.

Pour the mixture into the pie crust.

Bake for 45 to 55 minutes, or until the top is golden and the center is just set.

Cool completely on a wire rack.

Refrigerate for at least 2 hours before slicing and serving.