1. Make the crust: In a food processor, pulse flour, sugar, and salt until combined. Add butter and pulse until mixture resembles coarse meal, with a few pea-size pieces of butter remaining. Sprinkle with 3 tablespoons ice water. Pulse until dough is crumbly but holds together when squeezed (if necessary, add up to 2 tablespoons ice water, 1 tablespoon at a time); do not overmix. Form dough into a disk, wrap tightly in plastic, and refrigerate until firm, 1 hour.

2. Preheat oven to 375 degrees. On a floured surface, roll out dough to a 12-inch round. Place in a 9-inch pie plate, fold overhang under, and crimp edges. Prick dough all over with a fork. Place a sheet of parchment paper over dough and fill with pie weights or dried beans. Bake until edge is light golden, about 15 minutes; remove parchment and weights.

3. Make the filling: In a large bowl, whisk together flour, eggs, and brown sugar until smooth. Whisk in buttermilk, nutmeg, and cinnamon until combined. In a small saucepan, heat butter over medium, swirling pan occasionally, until golden brown and most of the foam has subsided, 8 to 10 minutes. Immediately whisk into buttermilk mixture. Pour filling into pie shell and bake until set but still slightly wobbly in center, 25 to 35 minutes. Let cool completely on a wire rack, 2 hours, before serving.