Sift together icing sugar, all-purpose flour, cornstarch, and fine salt into a mixing bowl. Rub in cold, diced unsalted butter until sandy. Add beaten egg, mix until combined. Shape into a disc, wrap in plastic wrap, chill for 30 minutes.

Preheat oven to 160°C (320°F). Roll out dough to 3mm thickness. Transfer to tart pan, press into edges. Bake for 15-20 minutes until golden brown. Cool completely.

Preheat oven to 120°C (250°F). Combine heavy cream, whole milk, sugar, and egg yolks in a bowl. Whisk until smooth. Pour custard over cooled base and bake for 1 hour until just set.

In a saucepan, combine granulated sugar and water, stir until dissolved. Heat over medium-high without stirring until deep amber, about 10-15 minutes. Pour over baked custard, cool slightly.

Allow tart to cool to room temperature, then refrigerate for at least 2 hours to set. Serve by slicing gently.