In a medium saucepan, whisk together the cornflour and the vegan milk then place over a low heat. Stirring constantly, bring the mixture to a simmer. Once it reaches around 70c the mixture will thicken quickly. Remove from the heat and immediately whisk in the sugar and the vegan butter. It should resemble a thick custard. Set aside to cool slightly for around 10 minutes.
In the bowl of a stand mixer fitted with a dough hook, add the flour, yeast and salt. mix to combine. Once cooled slightly, add the cornflour/milk mixture to the flour. Turn on the mixer and knead with the dough hook on medium speed until a ball of uniform dough forms. Cover with a damp cloth and leave for 10 minutes.
After the dough has rested, turn the mixer back on and knead for around 7-10 minutes. Turn off the mixed, remove the bowl and cover with a damp cloth again. Leave for 1.5 hours.
After the dough has proved, lightly flour a surface and turn out the dough. Punch out any large air bubbles and divide the dough into 9 pieces (they should be roughly 150g each). Working one piece at a time, fold the edges of the dough into the centre forming a sort of upside down jelly-fish shape. Turn it right way up, so you have a nice smooth topped piece of dough and using your little fingers and the sides of your palms, rotate the dough while tucking the bottom under to form a nice smooth ball. Repeat with the other pieces of dough and place them all on a baking sheet lined with parchment. Cover with the cloth and leave to prove for an hour.
Preheat the oven to 200c. Mix together your egg wash and, using a pastry brush, cover the tops of each bun with the egg wash. Sprinkle with seeds and place in the oven. Bake for around 15-18 minutes. Keep an eye on the buns as the tops can burn easily - be ready to take them out at the 15 minute mark.
Allow the buns to cool fully, or for at least half an hour in a cool space, before slicing, filling and eating. If desired, you can toast the sliced bun before filling.