Start by thoroughly greasing a 1 - 1.2 litre pudding basin. Place a disc of baking parchment in the bottom to prevent the top of the pudding from sticking. Pop the kettle on.
Mix together the ground flaxseed and water in a small bowl to make a flax egg; set aside to go gloopy.
Place the vegan butter/dairy free margarine, light brown soft sugar and maple syrup in a large pan over a low heat and stir until melted.
Mix in the soy milk, vanilla extract and the flax egg then sift in the flour, cocoa powder and bicarbonate of soda and whisk until no lumps of flour remain.
Pour the batter into the prepared basin and spread level.
Make a fold in the middle of a square of baking parchment and the same again in a square of tin foil.
Place the baking parchment on top of the pudding basin with the fold in the middle, followed by the tin foil. Press the foil over the edge of the pudding basin.
Tie a length of string very tightly around the foil – there should be a lip on the pudding basin; tie the string just under that. Tie a length of string across the top of the basin to make a handle. Trim off the excess foil and parchment, leaving at least a centimeter below the string.
Place a saucer upside down in a large saucepan and place the pudding basin on top. Pour boiling water into the pan so that it comes about halfway up the pudding basin, try not to let it come above the level of the foil.
Place the lid on the pan and place over a low heat. Cook at a gentle simmer for one and a half hours. Check after an hour to see if the water needs topping up but resist the urge to take the lid off the pan too often.
While the pudding is steaming make the sauce. Place the chopped chocolate, soy milk, golden syrup and vanilla extract in a pan over a low heat and whisk gently until melted and smooth. Set aside and re-heat as needed.
Once the pudding is done, cut off the string and remove the tin foil and parchment. Place a serving platter on top and flip over. The pudding should drop out of the basin (you might need to give it a bit of a wiggle). If it is stuck fast, run a knife around the edge to loosen.
Serve right away topped with the chocolate sauce and some ice cream or custard.