Melt butter in a small saucepan over medium heat. Cook 2-3 minutes or until butter is light brown and smells nutty. Keep a close eye on the butter as it cooks—it can go from just right to burned in seconds. Remove from heat.
In a large bowl, combine flours, baking soda, and salt, stirring with a whisk.
In a medium bowl, whisk sugar and melted butter until well blended. Stir in banana, yogurt, and eggs. Add to flour mixture; stir just until moist. Stir in walnuts.
In a small bowl, combine 1 cup batter and chocolate. Spoon plain batter alternately with chocolate batter into a 9 x 5-inch loaf pan coated with cooking spray. Swirl batters together using a knife (voila—it’s marbled!). Bake for 1 hour and 15 minutes or until a wooden pick inserted in the center comes out clean. Cool for 10 minutes in the pan on a wire rack; remove from the pan. Cool completely on a wire rack.