Martha's Buttermilk Pie

Preheat the oven to 325 degrees.

Combine sugar and flour in a large bowl. Add yolks and eggs, and whisk to combine. Whisk in butter, buttermilk, cream, zest, and vanilla until thoroughly combined. Pour into baked crust. Bake until filling is set but still wobbly, 45 to 55 minutes. Let cool to room temperature before serving.