Preheat oven to 350 degrees F. Lightly butter and dust with flour a 9×5 loaf pan.
In a large bowl, mash very ripe bananas with sugar. Then whisk in melted butter, eggs, vanilla, and buttermilk.
In a small bowl, stir together flour, baking powder, baking soda, and salt. Stir dry ingredients into wet ingredients.
In a microwave-safe bowl, heat chocolate in 20 second bursts until it’s just melted. Stir 1 cup of bread batter into the chocolate.
Pour half of the plain batter into your prepared loaf pan. Top with 1/2 the chocolate batter. Cover with plain batter and top with the last 1/2 of the chocolate batter. Use a chopstick to swirl the batters together.
Bake loaf at 350 degrees for 45-50 minutes until a tester comes out clean from the center.
Let loaf cool for a few minutes, then remove from pan. Serve warm or let cool completely and wrap tightly to keep fresh.