1. Preheat the oven to 200C (180C fan-forced). Grease and line a 20cm square baking tin with baking paper. Set aside.

2. Place the flour, icing sugar and salt in a food processor and process for a few seconds to combine. Add the butter and process until crumbly, then add the vanilla and pulse a few more times to bring the dough together.

3. Press the dough into the bottom of the baking tray, using the back of a spoon to even it out. Bake in the preheated oven for about 20 minutes or until pastry is light golden brown, then remove from oven and brush all over with the lightly beaten egg white. Return to oven for 1 minute to '"seal" the pastry – this prevents it from getting soggy when the topping goes on later. Set aside to cool, leaving the oven switched on.

4. For the topping, place the egg, sugar, vanilla seeds and salt in a large mixing bowl and whisk to combine, then use a rubber spatula to fold in the flour and orange zest. Finally, fold in the sliced rhubarb. Scrape the mixture onto the cooled, cooked pastry base and spread evenly all over.

5. Return to the oven to bake for 45 to 50 minutes, or until set. Cool completely before slicing into bars.

Note: They are best eaten on the day they are made, but may be kept for 2 days. The rhubarb will discolour a little, but this will not affect the taste.