Heat the waffle iron. Mix the soya milk with the vinegar and oil, then whisk in the sweet potato mash. Tip the polenta, flour and baking powder into a bowl, mix and make a well in the centre. Add a large pinch of salt, then slowly pour in the milk mixture and whisk to make a batter. Stir in half the chives.
Pour enough batter into the waffle iron to fill and cook for 4-5 minutes. Lift out the waffle, keep it warm and repeat with the remaining mixture until you have six waffles.
Meanwhile, mix the maple syrup with the soy sauce. Brush it over the mushrooms and season with pepper. Heat a little oil in a frying pan and fry the mushrooms on both sides until they are browned and cooked through – make sure they don’t burn at the edges. Serve the waffles topped with mushrooms, add a spoonful of soya yogurt, if you like, and scatter over the remaining chives.