Prep. Preheat oven to 350°F. Grease a 9x4-inch loaf pan and set aside. If desired, line pan with parchment paper.
Wet Ingredients. In a large bowl, mash together bananas and brown sugar. Mix in eggs and sour cream until well combined. *The batter may look curdled.
Dry Ingredients. In a smaller bowl, whisk together flour, baking soda, salt, and baking powder.
Mix Wet and Dry. Add dry ingredients to wet mixture. Mix until just combined.
Separate Batter. Portion out 250 grams of batter into a separate bowl. Add black sesame powder and molasses. Mix until well incorporated.
Layer Batter. Alternate adding layers of banana batter and black sesame batter into the prepared loaf pan. When finished, use a mini offset spatula or the back of a spoon to smooth the top of the batter into an even layer. If desired, sprinkle with white and black sesame seeds.
Bake. Bake for 50–60 minutes, rotating the pan halfway through, until a toothpick inserted in the center comes out clean. If the top browns too quickly, loosely tent with foil
Cool. Let the bread cool in the pan for 10 minutes, then unmold and transfer to a wire rack. Allow the bread to cool completely to room temperature before slicing with a serrated knife.