Preheat oven to 350 degrees F (175 degrees C). Grease a loaf pan.

Whisk flour, baking soda, and salt together in a bowl.

Whisk hot water and cocoa powder in a large bowl until smooth.

Combine mashed bananas, Greek yogurt, butter, brown sugar, eggs, and vanilla extract in another large bowl; mix well. Fold in flour mixture until thoroughly combined.

Mix half of the banana mixture into the cocoa mixture until well combined.

Spoon banana mixture and cocoa mixture into the prepared pan, alternating between the two.

Bake in the preheated oven until a toothpick inserted into the center comes out clean, about 55 minutes.