Instructions

for the dough

Grate 2 tablespoons butter on large holes of box grater and place in freezer. Cut remaining 8 tablespoons butter into ½-inch cubes.
Pulse ¾ cup flour, sugar, and salt in food processor until combined, 2 pulses. Add cubed butter and process until homogeneous paste forms, about 30 seconds. Using your hands, carefully break paste into 2-inch chunks and redistribute evenly around processor blade. Add remaining ½ cup flour and pulse until mixture is broken into pieces no larger than 1 inch (most pieces will be much smaller), 4 to 5 pulses. Transfer mixture to bowl. Add grated butter and toss until butter pieces are separated and coated with flour.
Sprinkle 2 tablespoons ice water over mixture. Toss with rubber spatula until mixture is evenly moistened. Sprinkle remaining 2 tablespoons ice water over mixture and toss to combine. Press dough with spatula until dough sticks together. Transfer dough to sheet of plastic wrap. Draw edges of plastic over dough and press firmly on sides and top to form compact, fissure-free mass. Wrap in plastic and form into 5-inch disk. Refrigerate dough for at least 2 hours or up to 2 days. Let chilled dough sit on counter to soften slightly, about 10 minutes, before rolling. (Wrapped dough can be frozen for up to 1 month. If frozen, let dough thaw completely on counter before rolling.)

for the pie

Roll dough into 12-inch circle on floured counter. Loosely roll dough around rolling pin and gently unroll it onto 9-inch pie plate, letting excess dough hang over edge. Ease dough into plate by gently lifting edge of dough with your hand while pressing into plate bottom with your other hand.
Trim overhang to ½ inch beyond lip of plate. Tuck overhang under itself; folded edge should be flush with edge of plate. Crimp dough evenly around edge of plate. Wrap dough-lined plate loosely in plastic wrap and refrigerate until firm, about 30 minutes. Adjust oven rack to middle position and heat oven to 350 degrees.
Line chilled pie shell with double layer of aluminum foil, covering edges to prevent burning, and fill with pie weights. Bake on foil-lined rimmed baking sheet until edges are set and just beginning to turn golden, 25 to 30 minutes, rotating sheet halfway through baking. Remove foil and weights, rotate sheet, and continue to bake crust until golden brown and crisp, 10 to 15 minutes longer. Transfer sheet to wire rack. (Crust must still be warm when filling is added.)
While crust bakes, whisk sugar, eggs, cornstarch, ⅛ teaspoon cinnamon, nutmeg, and salt together in bowl. Bring milk and cream to simmer in large saucepan over medium heat. Slowly whisk 1 cup of hot milk mixture into egg mixture to temper, then slowly whisk tempered egg mixture into remaining milk in saucepan. Cook over medium heat, whisking constantly, until mixture is thickened, bubbling, and registers 180 degrees, 30 to 90 seconds (custard should have consistency of thick pudding). Strain mixture through fine-mesh strainer into clean bowl, then stir in rum.
With pie still on sheet, pour custard into warm crust, smoothing top with clean spatula into even layer. Bake until center of pie registers 160 degrees, 14 to 18 minutes. Let pie cool completely on wire rack, about 4 hours. Spread whipped cream attractively over pie and dust with remaining ⅛ teaspoon cinnamon. Serve.

for the whipped cream

Whisk all ingredients together in bowl of stand mixer, cover with plastic wrap, and refrigerate for at least 4 hours or up to 24 hours. Whisk to combine before whipping.