Preheat oven to 425. Prepare muffin tin (grease, use non-stick spray or muffin cup liners)
Combine flour, baking powder & soda, salt, cinnamon, and nutmeg together in a bowl.
In a mixer (or large bowl), combine mashed bananas, melted butter, brown sugar, egg, vanilla and milk.
Add dry ingredients to wet ingredients and mix until combined.
Add raisins or nuts, if desired, and fold into batter.
Using an ice cream scoop, add batter to cupcake liners.
Add streusel topping to each muffin (topping instructions below).
Bake 5 minutes at 425, then lower heat to 350 for another 15 minutes.
Cool in the pan for 5-10 mites then transfer to wire rack.