Preheat oven to 350º F. Spray a 9-inch loaf pan with baking spray (such as Baker's Joy) or coat with butter and dust with all-purpose flour, being sure to shake out any excess flour. Set aside.
Mix bananas and sugar with electric mixer until well blended. Add butter and beat until smooth. Add eggs one at a time, fully incorporating after each egg.
Whisk flour, cocoa powder, and espresso powder in a medium mixing bowl until blended. Add flour/cocoa mixture to batter and mix until smooth. Add baking soda, salt, and vanilla extract and stir until well combined. Pour batter into prepared pan.
Bake until a wooden skewer comes out clean when inserted into the middle, about 1 hour.
Make-Ahead: Store chocolate banana bread under a cake dome, wrapped well in plastic wrap or in an airtight container. Freezer Friendly: Allow chocolate banana bread to cool completely. Wrap well in foil or plastic wrap and store in a zip top, freezer-safe bag for up to 3 months.