Preheat oven to 350 degrees.
To a bowl make the streusel topping by adding melted butter to brown sugar, flour, cinnamon and salt. Mix and place in the refrigerator for at least 20 minutes.
To a separate bowl add bananas and smash.
Add melted salted butter, sugar, egg, vanilla and beat.
Add flour, baking soda and salt and mix with a spoon until just combined.
Pour in a greased loaf dish and top the batter with the cold streusel topping.
Bake for one hour in the middle rack.
Cool for 20 minutes.