Cinnamon Streusel Banana Bread

Name: Cinnamon Streusel Banana Bread
Ingredients: 4 large very ripe bananas , mashed (about 2 cups ); 1/2 cup vegetable oil; 1/3 cup packed dark brown sugar plus 2 1/2 tablespoons , divided; 2 large eggs; 2 teaspoons vanilla extract; 1 cup white whole-wheat flour plus 2 tablespoons , divided; 1 cup all-purpose flour; 1 teaspoon baking soda; 3/4 teaspoon salt; 1/2 teaspoon baking powder; 1/4 cup coarsely chopped walnuts , toasted (see Tip); 2 tablespoons unsalted butter , melted; 2 teaspoons ground cinnamon