Preheat oven to 425°F.
In a large bowl mix the streusel ingredients, using your fingers or a pastry cutter to crumb the mixture. Set aside.
Spray a 6-count jumbo muffin pan with nonstick spray or use cupcake liners.
Whisk the flour, baking powder, baking soda, salt, cinnamon, and nutmeg together in a medium bowl. Set aside.
In a large bowl, mash the bananas.
On medium speed, beat or whisk in the melted butter, brown sugar, egg, vanilla extract, and milk.
Pour the dry ingredients into the wet ingredients, then beat or whisk until combined.
Fold in the nuts.
Add 3/4 cup of the batter into each liner.
Top with a generous amount of streusel.
Bake for 5 minutes at 425 then, keeping the muffins in the oven, reduce the oven temperature to 350°F.
Bake for an additional 20-25 minutes or until a toothpick inserted in the center comes out clean.
