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 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.
Transfer the dough to a lightly greased bowl, cover with clingfilm and either place somewhere warm to rise until doubled in size, about 1-2 hours, or in the fridge overnight.
While the dough is rising make the filling. Heat the butter, sugar, cocoa powder and milk in a pan until melted, stirring constantly. Remove from the heat, add the chocolate and stir until melted. Set aside until cooled and thickened slightly.
Tip the risen dough out onto a floured surface. Sprinkle with more flour and roll out to an approx. 30 x 40 cm rectangle. Spread over the filling evenly then fold the dough into thirds, like a business letter, you should end up with a 30 x 13 cm rectangle. Slide it onto a baking sheet and place in the fridge for about 20 minutes so that it firms up a little and is easier to work with.
Pat the dough out width-ways a little so that you end up with a 30 x 20 cm rectangle. Cut length-ways into 10 strips, each about 2 cm wide, a pizza cutter is ideal for this.
Line two baking sheets with baking parchment. Twist each strip from either end into a spiral, then shape into a coil, passing the end over the top and tucking it underneath. Place well spaced apart on the baking trays.
Loosely cover with oiled clingfilm and set aside to rise for about 45 minutes - 1 hour, until puffy. Meanwhile, preheat the oven to 180°C/160℃ fan/350°F/gas mark 4.
To make the glaze, mix the maple syrup and milk together in a small bowl.
Gently brush the risen buns with the glaze and bake for about 25 minutes, until deep golden and cooked through. Leave to cool on the trays for 10 minutes then transfer to a wire rack. Once cooled store in an airtight container.