1. Preheat the oven to 350°F (177°C) and lightly grease a 9 inch tart pan with a removable bottom. Place the pan on a baking sheet.

2. In a bowl combine the graham cracker crumbs, 1/3 cup sugar, 6 tablespoons melted butter, and 1/4 teaspoon fine salt. Stir until the mixture resembles damp sand.

3. Press the crumb mixture firmly into the bottom and up the sides of the tart pan, packing it tightly with the bottom of a measuring cup for an even crust.

4. Bake the crust for 10 to 12 minutes or until lightly golden and fragrant, then set aside to cool slightly while you prepare the filling.

5. In a mixing bowl beat the softened cream cheese until completely smooth with no lumps, then add 1/2 cup granulated sugar and beat until creamy.

6. Add the egg yolks one at a time, beating well after each addition so they fully incorporate and the mixture stays silky.

7. Whisk in the eggnog, cornstarch, vanilla extract, ground nutmeg, ground cinnamon if using, and a pinch of fine salt until the filling is smooth and well combined.

8. Pour the eggnog filling into the warm crust and smooth the top with a spatula, then tap the pan gently on the counter to release any air bubbles.

9. Bake the tart for 22 to 26 minutes, or until the edges are set and the center still has a slight wobble when you gently shake the pan. Do not overbake.

10. Remove the tart from the oven and let it cool completely on a wire rack, then refrigerate for at least 3 hours or until thoroughly chilled and firm.

11. To make the caramel shards, line a baking sheet with parchment paper. In a medium saucepan combine 1 cup sugar and 1/4 cup water and cook over medium heat without stirring until the sugar dissolves.

12. Continue cooking, swirling the pan occasionally, until the syrup turns a deep amber color. Immediately remove from the heat and carefully stir in 1 tablespoon butter and a pinch of flaky sea salt.

13. Quickly pour the hot caramel onto the prepared baking sheet and tilt the pan so it spreads into a thin, even layer. Let cool completely until hardened, then break into shards.

14. When ready to serve, remove the chilled tart from the pan and place it on a serving plate. Arrange caramel shards over the top and drizzle with caramel sauce or a little extra caramel from the pan.

15. Slice the tart with a sharp knife, wiping the blade between cuts for clean slices, and serve chilled with an extra sprinkle of nutmeg or whipped cream if desired.