Preheat the oven to 350℉.
In a bowl, combine the brown sugar, flour, cinnamon then mash in the cold butter until you have crumbles. Set that to the side.
Mash up the bananas then whisk in an egg, melted butter, vanilla and granulated sugar. In a separate bowl, mix together the all-purpose flour and baking soda. Combine the dry and wet ingredients, do not over-mix.
In a lined loaf pan, pour in half the banana bread batter. Then, scatter half the crumbles across the middle. Add the remaining batter on top of the crumbles. Then finish the top of the banana bread with the remaining crumbles.
Place in the oven and bake for 50 minutes or until the banana bread is cooked through. Ovens will vary, start checking around 40 minutes to be safe. You may need an hour or more depending on your oven.
When finished, allow to cool fully before cutting and serving, enjoy!