Make the Tart Crust

1. Preheat Oven: Preheat your oven to 350°F (175°C). Grease a 9-inch tart pan with a removable bottom.
2. Mix Dry Ingredients: In a mixing bowl, combine the flour, powdered sugar, and salt.
3. Cut in Butter: Add the cold, cubed butter to the flour mixture and use a pastry cutter or your fingers to blend until the mixture resembles coarse crumbs.
4. Add Egg Yolk: Stir in the egg yolk and add cold water, one tablespoon at a time, mixing until the dough comes together.
5. Chill Dough: Form the dough into a disk, wrap it in plastic wrap, and refrigerate for 30 minutes.
6. Roll Out Dough: On a floured surface, roll out the chilled dough to fit your tart pan, then transfer it to the pan, pressing it into the bottom and sides.
7. Blind Bake Crust: Place parchment paper over the crust and fill with pie weights or dried beans. Bake for 15 minutes, then remove weights and parchment and bake for another 10 minutes or until golden brown. Allow to cool.

Prepare the Eggnog Custard Filling

1. Whisk Ingredients: In a mixing bowl, whisk together the eggs, granulated sugar, vanilla extract, nutmeg, cinnamon, and cornstarch until smooth.
2. Heat Eggnog: In a saucepan over medium heat, warm the eggnog until just steaming, then slowly whisk it into the egg mixture until well combined.
3. Fill the Crust: Pour the custard filling into the cooled tart crust.

Bake the Tart

1. Bake Tart: Bake the tart in the preheated oven for 25-30 minutes, or until the filling is set but still slightly jiggles in the center.
2. Cool the Tart: Allow the tart to cool at room temperature for about 1 hour, then refrigerate for at least 2 hours or until chilled.