Preheat the oven to 350 degrees Fahrenheit (175 degrees Celsius).

In a medium bowl, whisk together the flour, cornstarch, powdered sugar, and a pinch of salt. Add the softened butter and egg yolk, and work them in with your hands until the dough comes together smoothly.

Use about half of the tart dough to press evenly into the bottom of a tart pan, then use the remaining dough to line the sides of the pan, taking your time to make it uniform and well-pressed.

Bake for 15 minutes. Remove from the oven and allow it to cool completely on a wire rack. If the crust puffs up in the oven, gently press the bottom of the crust down with the bottom of a flat glass.

In a medium saucepan, whisk together the sugar, cornstarch, and salt. Gradually add the milk and egg yolks, whisking until fully combined. Heat over medium heat, stirring constantly, and bring to a gentle boil. Once simmering, continue to stir frequently until custard thickens and then set a timer for 2 minutes to continue simmering to ensure that the cornstarch is fully activated.

Pour the custard through a fine-mesh strainer into a heatproof bowl. Immediately stir in the white chocolate chips, butter, and vanilla extract until smooth and fully melted.

Pour the custard into the cooled tart shell. Transfer the tart to the fridge and chill for at least 6 hours, preferably overnight, until fully set.

Just before serving, top the custard with sliced strawberries, blueberries, and raspberries. If desired, use a pastry brush to lightly brush the berries with thinned strawberry jam to give them a glossy finish.

Serve immediately.