Preheat oven to 180 degrees C and line or grease 12 muffin moulds.
In a large bowl mix together the oats, flour, baking soda, sugar and cinnamon.
To make the buttermilk put the vinegar in a measuring cup and fill up to a cup with soy milk. Set aside for 5 minutes.
Place the grated carrot and sweet potato in a clean cloth or paper towel and press to release as much liquid as you can.
When the buttermilk is thick, add it, the flax egg, sunflower oil, carrot, sweet potato and millet to the dry ingredients.
Fold to combine.
Add boiling water and stir to combine. Allow mixture to sit for 5 - 7 minutes.
Evenly spoon the batter in to the muffin moulds and bake in the oven for 25 minutes or until cooked through.
Combine maple syrup, cinnamon and sugar in a small bowl and brush over the top of each muffin.
Allow muffins to sit for 10 minutes before turning out on to a cooling rack.
Store in a covered container for up to 4 days.