Preheat the oven to 180°C/356°F (without fan).
Generously grease a 35 cm x 13 cm rectangular fluted tin with some butter.
Mix together the flour, cornflour, baking powder, sugar, salt, vanilla extract and butter in a food processor or KitchenAid.
The mixture should resemble damp sand.
If the mixture is too dry and crumbly, slowly add some fresh orange juice until the mixture just comes together in a ball. You may not need all of the orange juice.
You can also make the shortbread by hand by simply rubbing the butter into the dry ingredients until it is crumbly in texture, and then proceed to add any orange juice if necessary.
Roll the pastry into a ball.
Remove about ⅓ of the pastry and roll this into a rectangular shape, about 3 mm thick, which will be the lid of the tart. It is easiest to roll out of the pastry on a sheet of plastic warp or baking paper.
Use a cookie cutter in the shape of a snowflake or star to cut out some shapes in the lid.
Roll the remaining pastry (again on some plastic wrap or baking paper).
Line the tin with the pastry.
Trim off any excess pastry.
The pastry is likely to be quite soft but if any pieces fall off, you can simply squish some pastry into the tin to fill any gaps or holes.
Fill the tart with the date filling and spread it evenly.
Next, carefully cover the tart with the pastry lid.
Trim off any excess pastry and press together the pastry edges.
Brush the lid with some egg yolk and generously sprinkle with granulated sugar.
Bake the tart for about 25 to 30 minutes until it is lightly golden and the pastry is cooked underneath.
As a variation, instead of making a lid to cover the tart, you can simply use the pastry reserved for the lid by crumbling it over the filling for a crumble topping.