Preheat the oven to 375 degrees F (190 degrees C).
Get out an oven-safe non-stick frying pan. Roll out the Puff Pastry (200 gram) until about 3mm (1/8 inch) thick and cut into a disc slightly bigger than the pan. You can skip this step if you have ready-rolled puff pastry and just cut the disc. Leave in the fridge until needed.
Place the pan over a medium to high heat and add in the Rhubarb (6), Caster Sugar (125 gram), Unsalted Butter (2/3 cup), zest of the Sunkist Orange (1) and Vanilla Bean Paste (1 teaspoon). Mix well. Cook for 10 minutes to reach a good caramelization but do not stir the rhubarb.
After 10 minutes take off the heat and carefully cover the mixture with the prepared pastry. Tuck the sides in but be very careful not to burn your hands on the rhubarb toffee - it's hot! Prick the pastry with a fork several times as this will act as our steam release.
Whisk Egg (1) and brush the top of the pastry, this will ensure a well-browned crust.
Place into the oven and bake for 20 to 30 minutes, or until the pastry is perfectly cooked and the tarte has a good caramelization on the sides.
Remove from the oven and rest for 2 minutes.
Carefully invert the pan over a serving plate, letting the tarte drop gently. Serve immediately with a dollop of fresh Thickened Cream (200 gram) and Baby Basil (to taste).