1. Preheat the oven to 175°C. Lightly mist a 9-by-5-inch metal loaf pan with nonstick cooking spray and dust well with all-purpose flour to cover the pan completely, tapping out the excess.
2. Put the chocolate in a medium microwave-safe bowl and microwave on high in 30-second intervals, stirring, until melted and smooth, 1 minute to 1 minute 30 seconds. Set aside to cool slightly while preparing the batter.
3. Combine the bananas and sugar in a large bowl and mash with a potato masher or fork until mostly smooth with just a few small pieces of banana left. Add the oil and eggs and stir until combined. Using a wooden spoon or rubber spatula, mix in both flours, the baking powder, baking soda and salt. Stir in the buttermilk and vanilla.
4. Stir 1 cup batter into the melted chocolate. Fill the loaf pan with half the banana batter and then half the chocolate batter. Repeat the layers and gently swirl together using a spoon or knife. Bake until golden brown on top and a toothpick inserted in the center comes out clean, about 45 minutes.
5. Cool in the pan 15 minutes, and then turn out onto a wire rack to cool completely. Serve warm or at room temperature.
Cook's Note: To prepare your loaf pan for baking, you can also use a nonstick spray with flour already in it. Flouring the pan helps the bread rise well and prevents any collapse after it comes out of the oven.