In a large mixing bowl, combine flour, powdered sugar, cinnamon, nutmeg, and salt. Rub in the cold butter until it resembles coarse crumbs. Add the egg yolk and cold water, mixing until a dough forms. Shape into a disc, wrap in plastic, and chill for at least 30 minutes.

Preheat oven to 350°F (175°C). Roll out the chilled dough to fit your tart pan. Press into the pan, trimming edges. Prick the bottom with a fork, line with parchment, fill with weights, and bake for 15-20 minutes until golden. Cool completely.

Bloom gelatin by sprinkling it over 2 tablespoons of water and letting it sit for 5 minutes. Heat milk until warm. Stir in gelatin until dissolved, then add white chocolate and vanilla, stirring until smooth. Allow to cool to room temperature.

In a separate chilled bowl, whip heavy cream until soft peaks form. Once chocolate mixture is cool, gently fold whipped cream into it for a light filling.

Pour mousse filling into cooled tart shell, smoothing out with a spatula. Cover with plastic wrap and refrigerate for at least 4 hours.

Once set, decorate the tart with fresh berries, chocolate curls, and mint leaves. Use a warm knife to slice and serve chilled.