In a large mixing bowl, combine all-purpose flour, powdered sugar, cinnamon, nutmeg, and salt. Cut in the cold, cubed unsalted butter until the mixture resembles coarse crumbs. Add the egg yolk and 1-2 tablespoons of cold water, mixing until a dough forms. Shape it into a disk, wrap in plastic, and chill in the refrigerator for 30 minutes.

Preheat your oven to 350°F (175°C). Roll out the chilled dough to fit your tart pan. Press it gently into the pan, pricking the base to prevent bubbling. Line the crust with parchment paper, fill with pie weights, and bake for 25 minutes. Let cool completely before filling.

If using gelatin, bloom it in cold water for 5 minutes. Heat whole milk over medium heat until steaming. Remove from heat and stir in the gelatin to dissolve. Add the chopped white chocolate and stir until smooth. Allow to cool slightly.

Gently fold the whipped cream into the cooled chocolate mixture using a spatula. Start with a scoop of whipped cream to lighten the chocolate mixture, then gradually fold in the rest until well combined and airy.

Pour the mousse into the cooled tart shell and smooth the top. Cover with plastic wrap and refrigerate for at least 4 hours, preferably overnight.

Garnish your tart with fresh berries or chocolate curls. Slice the tart with a warm knife for clean cuts and serve chilled.