Preheat the oven to 350° and line a 9" x 5" baking tin with parchment paper (it sticks better to the pan if you spray the pan down with some baking spray or grease with a bit of butter first). If you let some parchment hang over the edges it makes lifting the bread out of the pan much easier.
Lug out the food processor, plunk in the peeled bananas—they'll be virtual mush at this point—and pour in the butter, sugar, and salt. Puree for 10 seconds until combined.
Crack in the eggs and add the sour cream to the processor and pulse just 3 times to start to combine them into the buttery-banana base.
Sift the flour, cocoa powder, baking soda, and espresso powder (yes, you have to... you don't want clumps of cocoa powder, trust me) either straight into the processor (carefully) or into a separate bowl (which you'll pour into the processor). Pulse this together just 5 more times.
Remove the lid and the blade from the processor and fold in just half of the chocolate chips with a rubber spatula, scraping the sides and bottom of the bowl as you do so to make sure everything combines, but without over mixing—in all, probably 5 folds.
Pour the dark batter into the prepared loaf tin and bake 50 - 60 minutes, until the center is softly set. Let it cool outside of the oven for about 15 minutes before removing it from the tin (by grabbing and lifting on those parchment hangovers) and letting it cool on a wire wrack either for a few more minutes or until completely cooled to room temperature. It's delicious at any temperature, really.
The cocoa must absolutely be Dutch-processed (I prefer Saco Foods brand). If you use natural cocoa powder you won't get the same results as it will react differently with the baking soda and sour cream.
For the cocoa powder, I love a mix premium Dutch-processed cocoa powder by Saco Foods with King Arthur's Black Cocoa powder—so intense.