Preheat the oven to 350°F (175°C).

In a mixing bowl, combine the flour and sugar for the crust. Add the cold cubed butter and mix until the mixture resembles coarse crumbs.

Stir in the egg yolk and add cold water, one tablespoon at a time, until the dough comes together.

Roll the dough out on a floured surface to fit the tart pan. Press the dough into the pan and trim any excess.

Line the crust with baking paper and fill it with pie weights or dried beans. Bake for 15 minutes, then remove the weights and bake for an additional 10 minutes or until golden brown. Allow to cool.

In a saucepan, heat the cream and milk over medium heat. In a separate bowl, whisk together the egg yolks, sugar, and cornstarch until smooth.

Once the cream mixture is hot but not boiling, gradually whisk it into the egg mixture to temper the eggs. Return the mixture to the saucepan and cook over low heat until it thickens, stirring constantly. Remove from heat and stir in the vanilla extract.

Pour the custard filling into the cooled tart shell and spread it evenly. Refrigerate for at least 2 hours until set.

Once the custard is set, arrange the fresh mixed berries on top of the tart. Garnish with mint leaves, if desired.

Slice and serve chilled.