Preheat your oven to 350°F (175°C).

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

Stir in the egg yolk and icy cold water until the dough comes together. If it's too crumbly, add a little more water, one teaspoon at a time.

Roll the dough out on a lightly floured surface into a circle about ¼ inch thick. Carefully transfer it to the tart pan, pressing it into the edges. Trim any excess dough hanging over the edges.

Place baking paper over the crust and fill it with pie weights or dried beans. Bake for 15 minutes. Remove the weights and paper, then bake for an additional 15 minutes until golden. Let it cool completely on a cooling rack.

In a saucepan, combine heavy cream, granulated sugar, and vanilla extract. Heat over medium heat, stirring until the sugar dissolves.

In a small bowl, mix cornstarch with 1 tablespoon of water to make a slurry. Slowly whisk the slurry into the heated cream mixture, stirring constantly until it thickens (about 2-3 minutes). Remove from heat.

Pour the cream filling into the cooled tart shell and smooth the top with a spatula. Allow it to chill in the refrigerator for at least an hour to set.

Once set, gently arrange the mixed berries on top. Garnish with fresh mint leaves if desired before serving.