Preheat the oven to 350°F (175°C). In a mixing bowl, combine the flour, powdered sugar, softened butter, and salt. Mix until the dough is smooth and well combined.

Press the dough into the bottom and up the sides of a tartlet pan or individual tartlet molds. Prick the bottom with a fork to prevent bubbling.

Bake the crusts in the preheated oven for 15-20 minutes or until lightly golden. Remove from the oven and let cool completely.

In a small saucepan, heat the heavy cream over medium heat until it begins to simmer. Remove from heat and add the chopped white chocolate, stirring until melted and smooth. Stir in the vanilla extract.

In a separate bowl, beat the egg whites until soft peaks form. Gradually add the granulated sugar, continuing to beat until stiff peaks form.

Gently fold the melted white chocolate mixture into the beaten egg whites until fully combined. Be careful not to deflate the mixture.

Spoon the mousse into the cooled tartlet crusts and refrigerate for at least 2 hours to set.

Before serving, garnish with fresh berries if desired.