Preheat the oven to 350°F (175°C).

In a large bowl, combine the flour, powdered sugar, and salt. Add the chilled butter and mix until the mixture resembles coarse crumbs.

Stir in the egg yolk and cold water, mixing until the dough comes together. If needed, add more water, one teaspoon at a time.

Press the dough into a 9-inch tart pan evenly across the bottom and up the sides. Prick the bottom with a fork.

Bake for 20-25 minutes or until the crust is lightly golden. Remove from the oven and let it cool completely.

In a medium bowl, whip the heavy cream until soft peaks form. In another bowl, mix the mascarpone cheese, sugar, and vanilla until smooth.

Gently fold the whipped cream into the mascarpone mixture until well combined.

Spread the filling evenly into the cooled tart crust. Top with mixed berries, arranging them as desired.

Chill the tart in the refrigerator for at least 1 hour before serving to set the filling.