Preheat the oven. Set your oven to 350°F (175°C).
Prepare your loaf pan(s). Grease your loaf pan with butter or cooking spray.
Mash the bananas
Combine the wet ingredients; Add the melted butter, sugars, eggs, and vanilla to the mashed bananas. Mix well until everything is incorporated.
Whisk the dry ingredients; In a separate bowl, whisk together the flour, baking soda, baking powder, salt, and cinnamon.
Mix the wet and dry ingredients; Add the dry ingredients into the wet mixture and fold gently with a spatula.
Add the butterscotch chips; Toss the butterscotch chips in 1–2 tsp of flour to prevent them from sinking to the bottom. Gently fold them into the batter.
Prepare the streusel topping; In a small bowl, mix the cold butter, brown sugar, white sugar, cinnamon, flour, and chopped butterscotch chips until crumbly. Sprinkle the streusel over the top of the batter before baking.
Pour batter into the pan
Bake; Place the pan in the oven and bake for 55–65 minutes (for a large loaf) or 40–50 minutes (for mini loaves). Check for doneness with a toothpick—it should come out mostly clean with a few moist crumbs. Tent with foil halfway through if the top begins to brown too quickly.
Cool the bread; Once done, let the bread cool in the pan for about 10–15 minutes, then transfer to a wire rack to cool completely.