In a food processor, pulse the flour, sugar, and salt. Add cold, cubed butter and pulse until coarse crumbs form. Add the egg yolk and ice water, pulsing just until the dough comes together.

Press dough evenly into a 9-inch fluted tart pan. Freeze for 30 minutes. Line with parchment, fill with pie weights, and blind bake at 375°F (190°C) for 15 minutes. Remove weights and bake another 10-15 minutes until a golden brown fluted pastry crust forms. Let cool.

In a bowl, aggressively whisk the 5 egg yolks, 1/3 cup sugar, and cornstarch until pale and thick.

In a saucepan, heat the eggnog, heavy cream, vanilla, and 1/2 tsp nutmeg until steaming. Do not boil.

Slowly whisk the hot milk mixture into the yolks. Return everything to the saucepan. Cook over medium-low heat, whisking constantly, until the mixture bubbles and thickens into a smooth pale yellow spiced custard.

Pour the hot custard through a fine-mesh sieve directly into the cooled tart shell. Smooth the top and chill for at least 4 hours until fully set.

In a clean saucepan, combine 1 cup sugar and water. Boil over medium-high heat without stirring until it reaches a deep amber color (340°F/170°C).

Pour the hot caramel onto a silicone mat in a thin layer. Let cool completely, then crack into tall, jagged translucent amber caramel shards.

Remove the set tart from the pan and place on a round ceramic dessert plate. Press caramel shards upright into the custard and finish with a ground nutmeg dusting before serving.