Sift together the icing sugar, all-purpose flour, cornstarch, and fine salt into a mixing bowl. Cut the cold unsalted butter into chunks and rub it into the mixture until it resembles coarse breadcrumbs, about 5 minutes.

Add the beaten whole egg to the sandy mixture and mix gently until just combined. Shape the dough into a disc, wrap in plastic wrap, and chill in the refrigerator for 30 minutes.

Preheat your oven to 160°C (320°F) fan. Roll the chilled dough to about 3mm thick and transfer it into a tart pan. Bake for 15-20 minutes until golden. Allow it to cool completely.

In a saucepan, combine the whole milk and granulated sugar over medium heat. Stir until the sugar dissolves. Whisk together the egg yolks and vanilla extract, then gradually add the warm milk mixture.

Reduce the oven temperature to 120°C (248°F). Pour the custard mixture into the cooled base and bake for 40-45 minutes until set but wobbly in the center. Cool at room temperature and then chill for at least 2 hours.

For the caramel glaze, combine the granulated sugar and water in a saucepan. Cook without stirring until deep amber, about 8-10 minutes. Pour over the chilled custard and let it set.

Slice the tart into wedges and serve chilled with fresh berries or mint. Enjoy!