1. Preheat oven to 400 degrees F; spray a muffin pan with cooking spray, or place paper liners in each cup; set pan aside
2. If using fresh sweet potatoes, pierce the skin with a fork and cook them in a microwave for 5 to 10 minutes, or bake in the oven until soft, then peel
3. In a bowl, cream the butter and sugar until light and smooth; add eggs and mix well
4. Add cooked or canned sweet potatoes and milk; mix well 
5. Sift in flour with baking powder, salt, cinnamon and nutmeg, and stir until just combined, being careful not to overmix
6. Chill muffin batter for 1 hour in fridge
   
   

For crumble topping:

1. In a pan, cook butter until foamy and browned (it will smell aromatic and nutty); move to a bowl and place in the fridge to cool
2. In a medium bowl, add the rest of the crumble topping ingredients, then add the cooled brown butter
3. Stir with a fork until the mixture forms pea-sized clumps
   
   

Bake the muffins:

1. Fill muffin tins with batter until four-fifths of the way full
2. Top with a spoonful of crumble mixture
3. Bake for 16 to 20 minutes; turning the pan halfway through; muffins are done when a toothpick inserted into the center comes out clean
4. Let cool in the pan for 10 minutes, then serve and enjoy