Instructions

1. Put the flour, 1⁄2 tbsp. sugar, and 1⁄2 tsp. salt into a food processor and pulse to combine. Add 6 tbsp. of the butter and pulse until pea-size pieces have formed. Drizzle in 2-3 tbsp. ice water; pulse until dough just comes together. Turn dough out onto a floured surface; form into a ball. Flatten into a disk and wrap tightly in plastic wrap. Chill for at least 1 hour.
2. On a floured surface, roll dough into a 12" circle about 1⁄8" thick; transfer to a 9" pie pan. Trim dough, leaving a 1⁄2" overhang; fold edges under. Flute edges with fingers. Chill for 1 hour.
3. Heat oven to 325°. Pour water into a 4-quart saucepan to a depth of 1"; bring to a simmer over medium-low heat. Whisk together remaining sugar and salt, brown sugar, buttermilk, nutmeg, egg yolks, and egg. Set bowl over saucepan. Add remaining butter; cook, whisking frequently, until smooth and warm, about 5 minutes. Remove from heat; let cool for 5 minutes. Pour mixture into chilled pie shell. Bake until center is just set, 45-50 minutes. Transfer pie to cooling rack; let cool completely before serving, about 2 hours.