Combine flour, sugar, and salt in a bowl.
Cut in cold butter until mixture is crumbly.
Add cold water gradually and form dough.
Press dough into a tart pan and chill for 20 minutes.
Preheat oven to 350°F (175°C).
Prick the chilled crust with a fork and bake for 15–18 minutes until lightly golden.
Allow crust to cool completely.
Melt white chocolate gently using a microwave or double boiler; let cool slightly.
Whisk egg yolks and sugar until pale and thick.
Fold in melted white chocolate gently.
Whip heavy cream until soft peaks form.
Fold whipped cream into the chocolate mixture until smooth and airy.
Pour mousse into the cooled tart crust and smooth the top.
Refrigerate tart for 2 to 3 hours until set.
Decorate with fresh berries, chocolate drizzle, or powdered sugar before serving.