Preheat oven to 350° F. Prepare an 8 1/2 x 4 1/2-inch loaf pan by coating with cooking spray or lining with parchment.
Combine the flour, baking soda, and salt. Set aside.
In the bowl of a stand mixer with the paddle blade attached, cream butter and sugars together until light and fluffy (about 1 minute). Add banana, eggs, and buttermilk. Beat until blended. Add flour mixture and beat at low speed just until moist.
To melt chocolate, place chocolate chips in a medium sized microwave safe bowl and microwave at HIGH 1 minute or until almost melted, stirring until smooth. Cool slightly. Add 1 cup batter to chocolate, stirring until well combined.
Spoon chocolate batter alternately with plain batter into prepared pan. Swirl batters together using a knife.
Bake in preheated oven for about 1 hour and 15 minutes or until a wooden pick inserted in center comes out clean. Remove from pan and cool completely on wire rack.