Fit the pie crusts into two 9-inch pie pans and place them in the refrigerator or freezer to chill while you make the filling. You want them good and cold when the filling goes in.
Preheat the oven to 350°F.
In a bowl, whisk together the sugar, flour and salt until the flour breaks down into the sugar. Set aside.
Melt the butter in a pitcher or large measuring cup. Let it cool slightly. Using a pitcher makes it much easier to pour the filling into the shells later.
Crack each egg into a separate small bowl and check that it is good before adding it. Beat each one lightly, then add it to the melted butter, one at a time. Mix with a fork.
Add the sugar and flour mixture to the butter and eggs and stir to combine.
Switch to a whisk. Add the buttermilk and vanilla and whisk until smooth and well combined.
Pull the chilled crusts from the refrigerator and pour the filling in, dividing it evenly between the two pies.
Bake at 350°F for 45 minutes. Check at 30 minutes; if the tops are browning too quickly, lay a piece of foil loosely over them for the remaining bake time.
Remove from the oven. The centers will still look a little jiggly — that is normal. Cool completely on the counter for 2 to 3 hours before slicing. The pie sets up as it cools.
Serve plain or with a spoonful of homemade whipped cream. It is especially good with a cup of coffee.