To make the tangzhong, place the flour in a saucepan and gradually whisk in the milk to avoid lumps.
Place the pan over a medium/low heat and whisk constantly until the mixture has thickened to a paste/pudding-like consistency.
Scrape the tangzhong into the bowl of a stand mixer fitted with a dough hook and pour the rest of the cold milk on top. This will cool the tangzhong down so that the heat doesn’t kill the yeast.
Add the vanilla, sugar, salt and flour and finally the yeast. Stir until it forms a rough dough then set the stand mixer to a medium speed and mix until the dough is smooth and stretchy, this can take about 10 minutes.
Add the vegan butter and knead for about another 5 minutes until it is fully incorporated and the dough is smooth, elastic and no longer feels greasy. You may need to scrape down the sides of the bowl a couple of times.
The dough should still be soft and sticky, but it should pull away from the sides of the bowl cleanly. If it seems too wet then you can knead in a little extra flour, a tablespoon at a time. Be careful though, this is meant to be a soft dough and adding too much extra flour will make the bread dry.
Place the dough in a lightly oiled bowl, cover and place in the fridge to rise overnight, or in a warm spot for 1-2 hours, until doubled in size.
Whilee the dough is rising, heat the oven to 180℃/160℃ fan/350℉/gas mark 4. Cut the rhubarb into approximately 7cm long pieces and arrange them in a single layer in a roasting tin. Sprinkle with the sugar and pour over the orange juice. Roast for 10 minutes until softened but still holding their shape. Set aside to cool.
Punch down the risen dough and give it a brief knead to knock out any air bubbles. Divide it into 10 even pieces (use a scale for accuracy) and roll them into balls.
On a lightly floured surface, use your hands to flatten each ball into a disc about 10cm wide, then use your fingers or the base of a glass to flatten the middle section of the disc even further, leaving an approx 2 cm rim around the edge.
Place the discs well spaced apart on two lined baking sheets. Loosely cover and leave to rise until puffy, about 30-45 minutes. If you gently press the dough with your finger it should spring back slowly, and maybe leave a slight indentation. If it springs back quickly then it needs a little longer, and if the dough deflates then it is over-prooved so keep an eye on it!
While the buns are rising make the frangipane. Stir together the ground almonds, flour, cornflour, baking powder and sugar. Add the melted butter, milk and almond extract and stir with a balloon whisk until combined.
Once the buns are done rising, divide the frangipane between them, spooning it into the hollows and spreading it level.
Top the frangipane with the rhubarb, reserving the juices for later. Mix together the milk and maple syrup then gently brush this mixture on the exposed rim of dough. Sprinkle with demarera sugar.
Bake the buns for 20-25 minutes until nicely browned.
While the buns are baking, pour the reserved rhubarb juice into a small pan and bring up to a simmer. Cook for a few minutes until it has reduced slightly then remove from the heat.
Gently slide the baked buns onto a wire cooling rack and brush the rhubarb with the syrup. Sprinkle with chopped pistachios if you like and serve asap.