Combine flour, powdered sugar, and salt in a food processor. Add cold butter and pulse until it resembles coarse crumbs. Add egg yolk and pulse until dough forms. Press into a 9-inch tart pan. Freeze for 30 minutes.

Preheat oven to 375°F (190°C). Line the frozen crust with parchment paper, fill with pie weights, and bake for 15 minutes. Remove weights and parchment, then bake for another 10-12 minutes until golden. Cool completely.

In a saucepan, warm the eggnog over medium heat. In a separate bowl, whisk together 4 egg yolks, granulated sugar, and cornstarch. Slowly whisk half of the warm eggnog into the yolk mixture to temper it.

Pour the tempered mixture back into the saucepan. Cook over medium-low heat, whisking constantly, until the custard thickens (5-7 minutes). Do not boil. Remove from heat and stir in vanilla and nutmeg.

Pour the filling into the cooled crust. Bake at 325°F (165°C) for 18-22 minutes, until the center is just set. Let it cool completely on a wire rack.

Refrigerate the tart for at least 4 hours, or overnight, to set completely.

Before serving, whip the heavy cream with 2 tbsp powdered sugar until stiff peaks form. Top the tart with the whipped cream and a dusting of cinnamon or nutmeg.