Preheat your oven to 350°F (175°C). In a mixing bowl, combine flour and powdered sugar, then cut in softened butter until the mixture resembles coarse crumbs. Add the egg yolk and just enough cold water to form a smooth dough. Press this dough evenly into a 9-inch tart pan, prick with a fork, and bake for 15-20 minutes or until golden. Allow the crust to cool completely.

While the crust is cooling, chop your white chocolate and melt it gently in a microwave or double boiler, stirring every 30 seconds until smooth. Let it cool to room temperature. In a separate bowl, whip the heavy cream until soft peaks form. In another bowl, beat the egg whites until frothy, gradually adding granulated sugar until stiff peaks form.

Gently fold the cooled white chocolate into the whipped cream, being careful not to deflate the mixture. Once combined, add a small portion of the beaten egg whites to lighten the mixture before integrating the rest. Carefully fold until no white streaks remain, ensuring the mousse remains airy and light.

Spoon the mousse into your cooled tart shell, smoothing the top with a spatula. Chill the tart in the refrigerator for at least 40 minutes.

After the mousse has set, garnish with fresh berries, chocolate drizzle, or mint. Slice into wedges and serve.