Prepare the Blueberry Sauce by whisking together cornstarch and lemon juice in a small bowl. In a saucepan over medium heat, combine fresh blueberries and sugar, crushing the berries slightly, and cook for about 5 minutes until juicy. Stir in the cornstarch mixture and cook for an additional 2-3 minutes until thickened. Set aside to cool.

Make the Shortbread Crust by preheating your oven to 180°C (350°F). In a large mixing bowl, combine melted unsalted butter, granulated sugar, vanilla extract, and a pinch of salt until smooth. Gradually mix in all-purpose flour to form a thick dough. Press the dough into a tart pan evenly. Bake for approximately 15 minutes until golden brown, then remove from oven to cool slightly.

Prepare the Filling by whisking together condensed milk, freshly squeezed lemon juice, lemon zest, and egg yolk in a medium bowl until well blended. Set aside.

Assemble the Tart by pouring the lemon filling evenly into the tart shell. Spoon the blueberry sauce over the filling in dollops and swirl gently with a knife to create a marbled effect.

Bake the assembled tart for another 17-19 minutes, observing the center which should be just set but slightly soft. Let cool at room temperature.

Cool and Chill the tart completely on a wire rack for about 30 minutes, then refrigerate for at least 2 hours before serving.