Place unbaked pie shell in a 9" pie pan. Trim, fold over, and flute edges. Chill fo 1 hour.
Heat oven to 325 degrees.
Pour water to depth of 1" into a 4 quart saucepan. Bring to simmer over medium-low heat.
Whisk together sugar and salt, brown sugar, buttermilk, nutmeg, egg yolks, and egg. Set bowl over saucepan. Add butter; cook, whisking frequently, until smooth and warm, about 5 minutes. Remove from heat; let cool for 5minutes. Pour mixture into chilled pie shell. Bake until center is just set, 45-50 minutes. Transfet pie to cooling rack; let cool completely before serving, about 2 hours.