1. Position a rack to the lower third of the oven and preheat it 350 degrees. Spray a 9-by-5-inch loaf pan with cooking spray and line it with a strip of parchment paper or aluminum foil about 8 inches wide to create a sort of "sleeve" that will make removing the loaf easier later.
2. To make the chocolate syrup, whisk together the sugar, cocoa powder, hot water, agave nectar and salt. Set the pot over high heat and bring the syrup just to a simmer, stirring occasionally until the syrup is smooth. Remove the pot from the heat and set aside to cool.
3. In a medium bowl, whisk together the flour, baking soda and salt.
4. In the bowl of an electric mixer, beat together the peanut butter and butter on medium speed until creamy. Add the sugars and beat until light and fluffy, about 2 minutes. Beat in the eggs one at a time. Scrape down the bowl.
5. Whisk together the mashed bananas, sour cream and vanilla in a small bowl. Beat into the butter mixture.
6. Reduce the mixer speed to low and stir in the dry ingredients. When just a few streaks of flour remain, stop the mixer, add the chocolate chips and gently fold the batter until everything is incorporated.
7. Transfer about a third of the batter (a little less is better than too much) to a medium bowl. Add the chocolate syrup and stir until well-blended.
8. Spread half the banana batter into the bottom of the prepared pan. Top with half the chocolate batter. Use a spoon to scoop and swirl the batter. Repeat with the second half of both batters.
9. Bake until a toothpick comes about clean but not dry (a few moist crumbs is ideal), about 75 to 85 minutes. Let cool in the pan on a wire rack for about 15 minutes before removing the loaf from the pan to cool completely.