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1. Preheat your oven to 175°C (350°F). Grease and flour a 23x13 cm (9x5 inch) loaf pan, or line it with parchment paper with overhanging edges for easy removal.
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2. In a small bowl, whisk together the cocoa powder and hot water until smooth and glossy. Stir in the dark chocolate chips while the mixture is still warm, letting them melt fully. Set aside to cool slightly.
0.3 cup Dutch-process cocoa powder,
3 tbsp Hot water,
0.5 cup Dark chocolate chips
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3. In a medium bowl, whisk together the flour, baking soda, baking powder, and salt. Set aside.
1.8 cup All-purpose flour,
1 tsp Baking soda,
0.5 tsp Baking powder,
0.5 tsp Salt
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4. In a large bowl, beat the softened butter with the granulated sugar and brown sugar on medium speed for 2–3 minutes until light and fluffy. Add the eggs one at a time, beating well after each addition, then mix in the vanilla extract.
0.5 cup Unsalted butter, softened,
0.8 cup Granulated sugar,
0.2 cup Brown sugar,
2 piece Eggs,
1 tsp Vanilla extract
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5. Mash the bananas until very smooth — no large lumps. Add the mashed bananas and sour cream to the butter mixture and mix on low until just combined.
3 piece Very ripe bananas,
0.5 cup Sour cream
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6. Fold in the flour mixture gently with a spatula in two additions, mixing just until no streaks of flour remain — do not overmix.
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7. Transfer roughly one-third of the banana batter into the bowl with the cooled chocolate mixture and fold together until fully combined to create the chocolate batter.
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8. Dollop alternating spoonfuls of the banana batter and chocolate batter into the prepared loaf pan. Once all batter is in the pan, use a skewer or butter knife to swirl through the batters 4–5 times in a figure-eight motion to create a marble effect — don't over-swirl or you'll lose the contrast.
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9. Bake at 175°C (350°F) for 55–65 minutes, until a skewer inserted into the center comes out with just a few moist crumbs. If the top browns too quickly, tent loosely with foil after 40 minutes.
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10. Remove from the oven and let the cake cool in the pan for 15 minutes, then lift out onto a wire rack to cool completely before slicing — this ensures a clean, tender crumb.
