To make the pie crust, in a mixing bowl, stir together the flour and salt, then toss in the cubes of cold butter to coat. Using your thumb and index finger, squish the pieces of butter to flatten, and continue to break the butter up until 50% of the butter is crumbly, but not completely uniform in texture; you want there to be chickpea-size nuggets of butter still present. Add the cold buttermilk one tablespoon at a time, blending and gathering the mixture with your hands until a dough is formed. Shape the dough into a disk, wrap in plastic wrap and refrigerate for at least 30 minutes. While the dough is chilling, prepare the filling.
To make the filling, add the buttermilk, sugar, vanilla, salt and flour to a blender or food processor and blend until smooth. Let the filling rest while the dough chills. Have your pie dish ready.
Once the dough is chilled, roll it out on a lightly floured surface or between two sheets of greaseproof paper into a 50cm round. Wrap the dough around the rolling pin to help ease it into the pie dish, then crimp the edges into a pattern with your fingers or a fork. Place the crust in the freezer to harden for another 10-15 minutes.
Preheat the oven to 180°C/gas mark 4 and position a rack in the centre of the oven. Remove the unbaked crust from the freezer and bake for 12-15 minutes, or until the pie turns light golden brown. Remove from the oven and pour in the filling mixture. Bake for 25 to 30 minutes, or until the centre is set and doesn’t jiggle when shaken. Remove from the oven. I like to serve mine warm from the oven, but if you’d prefer it chilled, place it in the fridge for at least two hours or until ready to serve.
For the top layer, when it’s time to serve, add the sugar to a small saucepan. Cook over medium heat until the sugar begins to brown and bubble. Swirl the pan to help any uncooked sugar liquify and cook for a few more seconds, then pour it out over the cold buttermilk pie. Working quickly, carefully tilt the pie dish to evenly distribute the browned sugar over the top. Allow to cool and harden for about five minutes, and serve immediately by cracking the top and slicing. Top with fresh berries, if desired.