Preheat oven to 350F.
In a food processor, combine brown sugar, flour and salt and pluse a few times. Add eggs and whizz until well combined. Add vanilla, nutmeg, butter (if using) and buttermilk and process for about 20 seconds, until very smooth.
Pour into prepared graham cracker crust, taking care not to over-fill the pie.
Bake at 350F for 30-35 minutes, until just set.
Allow to cool completely on a wire rack.
Serve chilled or at room temperature.
Makes 1 pie; serves 8-10.
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