Preheat the oven to 350°F (175°C).
In a mixing bowl, combine the flour and powdered sugar. Add the cold cubed butter and use a pastry cutter or your fingers to blend until the mixture looks like coarse crumbs.
Add the egg yolk and 1 tablespoon of cold water. Mix until the dough just comes together, adding more water if necessary. Form the dough into a disk, wrap it in plastic wrap, and chill for 30 minutes.
Roll out the dough on a lightly floured surface to fit a 9-inch tart pan. Press the dough into the pan and trim the edges. Prick the base with a fork.
Bake the crust for 15-18 minutes or until golden. Remove from the oven and let it cool completely.
Melt the white chocolate gently over a double boiler or in short bursts in the microwave, stirring until smooth. Let it cool slightly.
Whip the heavy cream with vanilla extract and a pinch of salt until soft peaks form.
Gently fold the melted white chocolate into the whipped cream until smooth and combined.
Pour the mousse into the cooled tart shell and smooth the top. Refrigerate for at least 4 hours or until set.
Before serving, garnish with white chocolate shavings and fresh berries if desired.