Preheat your oven to 200°C bake. Grease your muffin tin with either a bit of butter or non-stick spray - you can use either a 6 muffin tray (Texas-sized) or 12 (regular-sized), depending on whether you like big or medium-sized muffins!
Combine Chelsea Soft Brown Sugar, oil, egg, vanilla essence and milk in a mixing bowl. Mix well. Chop rhubarb into 2-3cm long chunks and add them to the bowl.
In a separate bowl sift dry ingredients together then blend the rhubarb mixture into the dry ingredients. Try not to over stir it. Fill the muffin tins three-quarters full.
In a separate bowl combine cinnamon, melted butter and Chelsea Demerara Sugar, then spoon on top of each of the muffins evenly. Bake for 20 - 25 minutes.