Preheat the oven to 180 degrees C. Grease muffin tins to hold 12 muffins.
Whisk together brown sugar, oil, vanilla and eggs in a small bowl.
Mix together the flour, baking powder, spices, salt and grated sweet potatoes in a large bowl. Make a well in the centre and pour in the egg mixture. Stir the egg mixture, gradually incorporating it with the flour mixture. Stir in the raisins and walnuts.
Spoon the batter into the tins. I like to fill each tin to the rim to make a large cap. Bake the muffins for 25 to 30 minutes or until a toothpick inserted into the middle comes out clean. Take the muffins out of the oven and run a paring knife carefully around each muffin, then invert the pan and knock one edge against your work surface to release the muffins. Serve the muffins right away.