How to Make It
In a medium bowl, whisk the flour with the sugar and salt. Add the butter and, using your fingertips, rub it into the flour until the mixture resembles coarse meal with some pea-size pieces remaining. Stir in the ice water just until the dough comes together. Form the dough into a ball, then flatten it into a 1-inch-thick disk. Wrap in plastic and refrigerate until firm, at least 1 hour or up to 1 day.
On a lightly floured work surface, using a lightly floured rolling pin, roll out the dough to a 12-inch round. Ease the dough into a 9-inch pie plate and tuck any overhang under itself. Crimp decoratively. Freeze until firm, about 30 minutes.
Preheat the oven to 425°. Line the dough with parchment paper, then fill with pie weights or dried beans. Bake for about 25 minutes, just until the dough is set. Remove the parchment paper and weights; bake for 8 to 10 minutes longer, until light golden. Transfer the crust to a rack to cool slightly, about 10 minutes. Reduce the oven temperature to 375°.
In a large bowl, whisk the sugar with the flour, nutmeg and salt. Whisk in the eggs 1 at a time. While whisking constantly, slowly drizzle in the melted butter followed by the buttermilk, rum and vanilla. Pour the filling into the warm crust. Bake for 15 minutes, then reduce the oven temperature to 350° and bake for about 40 minutes longer, until the custard is just set at the edge and slightly jiggly in the center. Transfer the pie to a rack and let cool completely. Serve at room temperature or chilled.