Sift icing sugar, flour, cornstarch, and salt into a bowl. Rub in cold butter until mixture resembles coarse sand. Stir in beaten egg until cohesive dough forms. Wrap in plastic wrap and chill for 30 minutes.

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

Reduce oven to 120°C (248°F). Heat milk and sugar in a saucepan, stirring until sugar dissolves. Remove from heat and stir in vanilla. Whisk egg yolks, then slowly add warm milk mixture while whisking.

Pour custard mixture into prepared crust, filling almost to top. Bake for 40-50 minutes until set but still jiggles slightly. Cool on wire rack then refrigerate for at least 4 hours.

Combine granulated sugar and water in a saucepan. Bring to boil without stirring until deep amber color, about 10 minutes. Remove from heat and stir in a pinch of salt.

Once tart is chilled, pour caramel glaze over custard for an even layer. Allow to set for 15 minutes. Slice and serve, optionally garnished with fresh fruit or whipped cream.