Preheat the oven to 350°F (175°C). In a mixing bowl, combine the flour, powdered sugar, and salt. Add the softened butter and egg yolk. Mix until the dough comes together.

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 for 15-18 minutes or until lightly golden. Remove from the oven and let cool completely.

While the tart shells are cooling, melt the white chocolate in a microwave-safe bowl in 30-second intervals, stirring in between until smooth. Let it cool slightly.

In a separate bowl, whip the heavy cream with the vanilla extract and 1 tablespoon of powdered sugar until soft peaks form.

Gently fold the melted white chocolate into the whipped cream until fully combined.

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

Before serving, garnish with fresh berries if desired.