Preheat oven to 325 degrees F (165 degrees C). Grease a 9x5-inch loaf pan.
Combine butter and honey in a large bowl; beat with an electric mixer until well combined. Mix in bananas and vanilla extract.
Mix whole wheat flour, baking soda, baking powder, and fleur de sel together in a separate bowl. Mix into the butter mixture until batter is combined.
Whisk hot water into the batter. Fold in dark chocolate and walnuts. Pour batter into the prepared loaf pan.
Bake in the preheated oven until a toothpick inserted into the center of the loaf comes out mostly clean, about 45 minutes. Cool in the pan for 5 minutes; invert onto a baking rack to cool completely.
