Begin with the creme patisserie. Place the milk in a saucepan and set to a low heat. In a bowl beat the egg yolks, sugar, vanilla extract and cornflour until pale and fluffy.
When the milk has come to a simmer, pour over the egg mixture, whisking as you pour, you should end up with a smooth slightly thickened custard.
Now pour the mixture back into the saucepan and heat, beating continuously. The mixture will thicken substantially, continue to stir for a minute. Pour into a clean bowl and place a parchment disc directly on the surface of the custard to stop a skin forming. Cool completely before using. Once the creme patisserie has cooled take a balloon whisk or a hand electric whisk and beat until smooth. Set to one side.
Wash and cut the rhubarb into 4cm chunks. Place on a baking tray. Sprinkle over the caster sugar, zest of an orange and a tablespoon of the juice. Cover with foil and bake 160Cfan for 20-25 minutes or until just cooked through. Set to one side.
When the rhubarb has cooled, add one third to the custard (not the juice) and stir to break it up. Cover and set to one side.
Pour the milk into a small saucepan and heat to 38°C, remove from the heat and add the fresh yeast, whisk to incorporate (if using fast action yeast add it to the flour). Place the flour, cardamom, sugar and salt in the bowl of a free standing mixer with the dough hook attached. Add the milk mixture with the motor running. Mix until a dough forms on a slow speed. Once the dough has come together add three quarters of the egg and the butter, increase the pace to medium/high and work the dough for another 5-8 minutes or until it feels shiny and smooth.
Place the dough in a bowl and cover with a tea towel and let it rest for 20-30 minutes to prove.
When the dough is ready divide into two. Roll each piece into a rectangle on a lightly floured surface. They should be approximately 25-30cm long and 15-18cm wide. Place on a baking tray lined with parchment paper. Spread an even layer of filling in the centre of each rectangle. Place the remaining rhubarb on the creme patisserie, no juice. Now cut slits from top to bottom at an angle on each side. Start overlapping them, bringing them into the centre, until you reach the bottom. Cover with a clingfilm and leave to prove for approximately 45 minutes or until almost double in size. (this depends on the room temperature). Meanwhile, heat the oven to 175°Cfan.
Brush with the remaining egg (add a little milk if needs be) and sprinkle with sugar nibs and cardamom. Bake for approximately 20-25 minutes or until it’s golden brown in colour. Cool on a wire rack. Serve warm or at room temperature with coffee.