Preheat oven to 350F. Grease a 9-inch round cake pan, line the bottom with parchment, and grease the parchment. This cake is sticky and the parchment is non-negotiable.

In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt until no cocoa lumps remain. Set aside.

In a large bowl, mash the bananas thoroughly until mostly smooth with small lumps. Whisk in the sugar until combined, then whisk in the sour cream, eggs, oil, and vanilla until smooth.

Pour the dry ingredients over the wet. Stir with a rubber spatula until just combined with a few streaks of flour remaining. Fold in 3/4 cup chocolate chips. Do not overmix or the cake turns tough.

Scrape batter into the prepared pan and smooth the top. Sprinkle remaining 1/4 cup chocolate chips on top. Bake 45-55 minutes until a toothpick inserted in the center comes out with moist crumbs but no wet batter. The top will look slightly cracked and feel springy.

Cool in the pan 15 minutes, then run a knife around the edge and invert onto a wire rack. Peel off parchment and flip right-side up. Cool completely before slicing, about 1 hour. The texture sets as it cools.