Step 1: Pulse flour, powdered sugar, and cold butter in a food processor until it resembles coarse crumbs. Add the egg yolk and pulse until the dough just comes together.
Step 2: Press the dough into a 9-inch tart pan with a removable bottom. Chill in the freezer for 30 minutes.
Step 3: Preheat oven to 375°F (190°C). Line the tart with parchment paper and pie weights. Blind bake for 15 minutes, then remove weights and bake for another 10 minutes until golden. Let cool.
Step 4: Lower oven temperature to 300°F (150°C). In a medium saucepan, heat heavy cream until it just begins to simmer.
Step 5: In a large bowl, whisk together 5 egg yolks, granulated sugar, pumpkin puree, vanilla, cinnamon, nutmeg, and salt.
Step 6: Slowly stream the hot cream into the pumpkin mixture while whisking constantly to temper the eggs.
Step 7: Strain the mixture through a fine-mesh sieve into the cooled tart shell to ensure a smooth custard.
Step 8: Bake for 30-35 minutes or until the edges are set but the center still has a slight wobble.
Step 9: Cool to room temperature, then refrigerate for at least 4 hours or overnight.
Step 10: Just before serving, sprinkle caster sugar evenly over the top and caramelize with a kitchen torch.