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

Add the egg yolk and mix until the dough forms a ball. If the dough is too dry, add a teaspoon of cold water.

Roll out the dough on a floured surface to about 1/8 inch thick. Cut into circles and press them into a muffin tin to form tartlet shells.

Prick the bottoms of the tartlet shells with a fork to prevent bubbling. Bake for 12-15 minutes or until lightly golden. Let cool completely.

In a microwave-safe bowl, melt the white chocolate chips in 30-second intervals, stirring in between until smooth. Let it cool slightly.

In a separate bowl, whip the heavy cream and vanilla extract until soft peaks form. Gently fold in the melted white chocolate until fully combined.

Add the milk to the mixture and stir until smooth. Spoon the mousse into the cooled tartlet shells.

Refrigerate for at least 2 hours to set. Before serving, garnish with fresh berries if desired.