Combine flour, powdered sugar, cinnamon, nutmeg, and salt in a mixing bowl. Cut in cold butter until resembling coarse crumbs. Mix in egg yolk and cold water to form dough. Wrap in plastic and chill for 30 minutes.

Preheat oven to 350°F (175°C). Roll out dough and fit into tart pan. Cover with parchment and fill with weights. Bake for 15 minutes, remove weights, and bake for another 10 minutes until golden. Cool completely.

Bloom gelatin in cold water for 5 minutes. Heat milk until steaming, stir in gelatin until dissolved, then add chopped white chocolate until melted and smooth. Stir in vanilla and cool slightly.

Whip heavy cream until soft peaks form. Gently fold into cooled white chocolate mixture in batches to maintain airy texture.

Pour mousse into cooled crust, smoothing the top. Cover and refrigerate for at least 4 hours to set.

Decorate the tart with berries, chocolate curls, or mint leaves. Use a warm knife for clean slices. Serve chilled.