Preheat oven to 350°F. Set a rack to the second-to-bottom position.
Grind walnuts coarsely in a food processor. Add graham crumbs and sugar; then pulse to blend while drizzling in melted butter. Press into a 9-inch pie plate.
Bake 10–15 minutes. Set aside to cool. Piecrust can be made up to 3 weeks in advance and frozen.
In a heavy-bottomed 2-quart pot, whisk whiskey or brandy, milk, and gelatin together over low heat, continuously until gelatin is completely dissolved, 2–3 minutes. Whisk in brown sugar, then eggs one at a time; continue whisking over low heat (don’t let it come to a boil) as you add pumpkin, spices, and salt. Cook 5–7 minutes, until custard is smooth and steaming.
Turn off heat and transfer custard to a large ceramic bowl to cool at room temperature. Don’t refrigerate, because you don’t want the custard to firm up.
In a separate bowl, whip cream to stiff peaks. When custard has cooled to room temperature (about 45 minutes), fold cream in gently. Fold filling into cooled pie shell; chill at least 2 hours. Garnish with walnut halves.