Preheat the oven to 350°F (175°C).
In a mixing bowl, combine the all-purpose flour, granulated sugar, and cold, cubed butter. Use a pastry cutter or your fingers to mix until the mixture resembles coarse crumbs.
Add the egg yolk and ice water to the mixture, mixing until it forms a dough. Wrap the dough in plastic wrap and refrigerate for 15 minutes.
Roll out the chilled dough on a lightly floured surface until it’s about ⅛ inch thick. Transfer it to the tart pan and gently press it into the bottom and sides. Trim any excess dough.
Line the crust with baking paper and fill it with pie weights or dried beans. Bake in the preheated oven for 15 minutes. Remove the weights and baking paper, and bake for an additional 10 minutes or until lightly golden.
While the crust cools, prepare the filling by whisking together the mascarpone cheese, powdered sugar, and vanilla extract until smooth and creamy.
Spread the mascarpone filling evenly over the cooled tart crust.
In a separate bowl, toss the strawberries, blueberries, and raspberries with honey. Decorate the tart with the berry mixture on top of the mascarpone filling.
Chill the tart in the refrigerator for at least 30 minutes before serving.