In another bowl, beat egg. Then add vanilla, oil, brown sugar and evaporated milk, whisk to blend.
Add the flour mixture, and mix well.
Add the sweet potato and stir in, until evenly distributed.
Spoon batter into baking cups, nearly full.
In a small bowl mix the topping ingredients. Sprinkle generously on the muffins, you should have the right amount for about a dozen standard size muffins.