Grease two baking tins with the vegan butter and set aside.
Melt the vegan butter for the dough. Add it to a large mixing bowl with the soy milk and caster sugar. Make sure the mixture is warm rather than hot.
Sprinkle the yeast evenly over the mixture and leave for 1 minute.
Add the plain flour, strong white bread flour and salt. Stir until the ingredients begin to come together.
Knead gently until a soft dough forms. Add orange food colouring, if using, and knead again until the colour is evenly distributed.
Shape the dough into a ball and return it to the bowl. Cover with a clean tea towel or plastic wrap and leave somewhere warm for approximately 1 hour, or until nearly doubled in size.
Meanwhile, preheat the oven to 165C / 325F / Gas Mark 3.
Make the filling by mixing together the vegan butter, soft brown sugar, cinnamon, ginger, nutmeg, cloves and allspice. Add black food colouring, if using, and mix until evenly coloured.
Once the dough has risen, turn it out onto a lightly floured work surface and knead gently until it is no longer sticky.
Roll the dough into a large rectangle approximately 1cm (½ inch) thick.
Spread the pumpkin spice filling evenly over the dough.
Roll the dough tightly from one long edge until you have a log. Pinch the seam closed and place the log seam-side down.
Trim the uneven ends. Cut the log in half, then divide each half into 6 equal pieces.
Arrange 6 rolls in each prepared tin.
Cover and leave somewhere warm for another 30 minutes, until the rolls have become noticeably puffier.
Remove the covering and bake in the centre of the oven for approximately 30 minutes, until golden brown.
While the rolls are baking, whisk together the icing sugar, soy milk and vanilla extract until smooth.
Drizzle the glaze over the rolls while they are still warm.
Serve and enjoy!