Preheat the oven to 350°F (175°C). Grease a 9×5 inch loaf pan and set aside.
In a large mixing bowl, combine the mashed bananas, melted butter, and granulated sugar. Mix well until smooth.
Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
In a separate bowl, whisk together the flour, baking soda, salt, and ground cinnamon.
Gradually add the dry ingredients to the banana mixture, alternating with the buttermilk. Begin and end with the dry ingredients. Mix until just combined; do not overmix.
Pour the batter into the prepared loaf pan and spread evenly.
To make the brown sugar streusel, combine the brown sugar, flour, and ground cinnamon in a small bowl. Add the cold butter pieces and use a pastry cutter or your fingers to mix until the mixture resembles coarse crumbs.
Sprinkle the streusel evenly over the banana bread batter in the loaf pan.
Bake in the preheated oven for 55-60 minutes, or until a toothpick inserted into the center comes out clean.
Remove from the oven and allow the bread to cool in the pan for 10 minutes.
Transfer the bread to a wire rack to cool completely before slicing and serving.