Step 1

Preheat oven to 350°. Lightly coat a 9x5" loaf pan with nonstick spray. Line bottom of pan with parchment paper, leaving a generous overhang on long sides.

Step 2

Whisk all-purpose flour, whole wheat flour, cocoa powder, baking soda, baking powder, and salt in a medium bowl.

Step 3

Mash 4 bananas with a potato masher or fork in a medium bowl until smooth. Using an electric mixer on medium-high speed (or a stand mixer fitted with the paddle attachment), beat brown sugar and butter in a large bowl until light and fluffy, about 4-5 minutes. Add eggs one at a time and beat until incorporated, scraping down sides of bowl if needed. Reduce speed to medium-low, add sour cream, vanilla, and mashed bananas; beat until combined. The mixture may look grainy and broken at this point, but don’t fret! It will emulsify when the dry ingredients are added.

Step 4

Reduce mixer speed to low and gradually add dry ingredients, beating just until incorporated (do not overmix or cake will become tough). Fold in chocolate.

Step 5

Scrape batter into prepared pan and smooth top. Slice remaining banana lengthwise into 3 equally thick planks and place on top of cake, cut sides up. Sprinkle batter, as well as bananas, with demerara sugar.

Step 6

Bake cake until a tester inserted into the center comes out clean (there may be some melted chocolate residue left on the tester, which is fine), 65–75 minutes.

Step 7

Transfer pan to a wire rack and let cake cool completely. Using parchment overhang, lift cake from pan and serve.