Moist Banana Bread with Crunchy Streusel Topping

Name: Moist Banana Bread with Crunchy Streusel Topping
Prep time: 10 min
Cook time: 1 h
Total time: 1 h 10 min
Cuisine: American
Ingredients: 2 cups all-purpose flour; 1 teaspoon baking powder; 1 teaspoon baking soda; 1/2 teaspoon salt; 1 teaspoon ground cinnamon; a dash of nutmeg; 1/2 cup unsalted butter (, softened to room temperature); 1/2 cup brown sugar; 1/2 cup white sugar; 2 large eggs (, at room temperature); 4 very ripe bananas (, mashed with a fork or potato masher); 1/4 cup buttermilk; 1 teaspoon vanilla extract; 1/2 cup walnuts (, toasted and chopped); 3 tablespoons flour; 1/4 cup brown sugar; 1/4 teaspoon cinnamon; 2 tablespoon unsalted butter (, slightly softened and cut into pieces); 1/4 cup chopped walnuts