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 cold milk on top. This will cool the tangzhong down so that it doesn't kill the yeast.
Add the sugar, espresso powder, salt, flour and 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, about 5-10 minutes. It is quite a soft, sticky dough, resist the urge to add extra flour. You should be able to stretch a section of the dough thin enough to see light through it without it breaking (the windowpane test).
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.
Place the dough in a lightly oiled bowl, cover and place in a warm spot to rise for 1-2 hours until doubled in size.
While the dough is rising make the filling. Heat the butter, sugar, cocoa powder, espresso powder, cinnamon 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.
Knead the risen dough very briefly to knock out any air bubbles then roll out on a lightly floured surface to an approx 38 x 48 cm/15 x 19 in rectangle.
Spread the chocolate filling evenly over the dough.
Use a pizza roller or a sharp knife to cut the rectangle lengthways into 12 strips. Gently roll the strips up into spirals, don't roll them too tightly.
Arrange the rolls spaced apart in a parchment lined 23 x 33 cm/9 x 13 in tin. Cover and set aside to rise for about 45 minutes until puffy. If you press one gently the dough should spring back slowly and leave a slight indentation. If it springs back quickly then they need a little bit longer. While they are rising preheat the oven to 180°C/160°C fan/350°F/gas mark 4.
Bake the rolls for 35-40 minutes until nicely browned and the internal temperature reaches at least 90°C/194°F on a probe thermometer.. If any of the middles have popped up you can gently press them back down again once the rolls are baked.
To make the glaze, sift the icing sugar into a bowl, add the espresso powder and stir in enough milk to make a pourable consistency. Drizzle the glaze over the buns and serve.