Preheat the oven to 350ºF.
Line a 9-inch loaf pan with parchment then grease the parchment with cooking spray.
Whisk together the flour, sugar, baking soda and salt in a bowl. Set aside.
Mix mashed bananas, buttermilk, eggs, melted butter and vanilla in a medium bowl.
Lightly fold banana mixture into dry ingredients with rubber spatula just until combined and batter looks thick and chunky. Stir in any desired add-ins.
Scrape batter into prepared loaf pan.
Prepare the streusel topping by combining all ingredients in a bowl and using a fork to cut the butter evenly into the dry ingredients.
Sprinkle the streusel topping over the unbaked loaf.
Bake the bread for 55 minutes or until a toothpick inserted comes out clean.
Allow the loaf to cool in the pan for 5 minutes then transfer it to a cooling rack to continue cooling. Slice and serve.