Preheat oven to 325°F (170°C). Spray an 8 1⁄2x4 1⁄2- inch loaf pan with baking spray with flour. Line pan with parchment paper, letting excess extend over all sides of pan.

In a medium bowl, whisk together flour, cocoa, salt, baking powder, and baking soda.

In a large bowl, whisk together banana, brown sugar, oil, sour cream, eggs, and vanilla. Gradually stir in flour mixture just until combined. Fold in chocolate chips. Spread batter into prepared pan.

Dip a clean small offset spatula in oil to coat lightly. Starting at one short side of pan, score a 3- to 4-inch line lengthwise along center of batter with oiled spatula.

Bake until a wooden pick inserted in center comes out clean and an instant-read thermometer inserted in center registers 205°F (96°C), 1 hour and 10 minutes to 1 hour and 20 minutes. Let cool in pan for 10 minutes. Using excess parchment as handles, remove from pan, and let cool completely on a wire rack. Garnish with confectioners’ sugar, if desired. Store in an airtight container for up to 3 days.