1Make the streusel topping: in a bowl, combine 1 1/2 TB salted butter, 1/3 C brown sugar, 2 TB flour, and 1/8 tsp cinnamon. Use a pastry cutter to mix until crumbly.
2Make the banana bread batter: mash 4 ripe bananas in a bowl.
3Add melted salted butter, sugar, egg, and vanilla to the mashed bananas and beat until combined.
4Add flour, baking soda, and a pinch of salt. Hand mix until everything is incorporated. Do not overmix.
5Pour batter into a greased loaf dish and top with streusel topping.
6Bake at 350°F for one hour.
7Slice and serve. Add a little salted butter to each slice.