Preheat oven to 350 degrees F. Be sure rack is placed in the center. Spray a 9 x 5 inch loaf pan with cooking spray. Line with parchment paper if desired.
In a large bowl, mash banana. Add pumpkin, cooled melted butter, brown sugar, white sugar, eggs, and vanilla. Whisk until well blended.
In a medium bowl, mix flour, baking soda, salt, cinnamon, pumpkin pie spice, and nutmeg. Add flour mixture to banana/pumpkin mixture.
Using a rubber spatula or wooden spoon, gently mix the batter until just combined. Don’t overmix the batter! 10-15 strokes is plenty. It’s ok if you have a few flour pockets visible.
Stir in nuts, if using.
Pour batter into prepared loaf pan. Smooth the top.
To make the streusel topping, combine cinnamon, flour, and brown sugar to a small bowl. Cut 6 tablespoons of very cold butter into cubes and add it to the flour/brown sugar mixture. Use a pastry blender, fork, two knives or your fingertips to blend mixture until it resembles coarse crumbs.
Spread streusel topping over batter.
Bake in center of oven for 50 minutes to 1 hour and 5 minutes.
Cool in pan for 5 minutes.
Remove from pan and cool completely on wire rack.