Preheat oven to 325 degrees. Prepare a 9x5 loaf pan with non-stick spray. You may also line it with parchment paper and spray the paper for easier removal.
In a large bowl, mash up the bananas with a fork or masher. Add in the eggs, buttermilk, butter, vanilla extract and whisk until everything is combined. Add in the brown sugar and mix until well combined.
Reserve a cup of the batter and melt the chocolate chips. Using a microwave, heat for 30 seconds and stir, then heat again for 15-20 seconds until the chocolate is melted. Pour the melted chocolate into the reserved 1 cup of batter and mix the two together.
Pour ⅓ of the plain batter into the batter of the prepared loaf pan and spread it to the edges. Pour ½ of the chocolate batter on top of the plain batter and spread it to the edges. Repeat with the plain batter, the chocolate batter and then ending with the plain batter. Use a knife and swirl through the pan to create a swirl design.
Bake for 60 minutes, or until a toothpick comes out clean. Cool for 10 minutes and then transfer the bread to a cooling rack to finish cooling.