In a mixing bowl, combine flour, salt, and sugar, then cut in chilled butter until resembling coarse crumbs.

Add egg and mix just until combined. Shape into a disk, wrap in plastic, and refrigerate for at least 1 hour.

Preheat oven to 190°C (374°F). Roll out dough to 1/4 inch thickness, fit into a tartlet pan, line with parchment, and bake for 10 minutes.

Remove weights, bake for an additional 5 minutes until golden brown, then cool on a wire rack.

Whisk together egg yolks and sugar until pale. Heat 1 cup of heavy cream until warm, then whisk into yolks.

Return to low heat, stirring continuously until thickened. Remove from heat and stir in chopped white chocolate until melted.

Whip remaining heavy cream to medium-stiff peaks. Gently fold in chocolate mixture until combined.

Spoon mousse into cooled tart shells, cover with plastic wrap, and refrigerate for at least 4 hours.

Before serving, garnish with fresh berries or dark chocolate. Slice and serve.