Prep the oven and pan: Preheat your oven to 175°C (350°F). Line a 9x5-inch loaf pan with parchment paper, leaving an overhang on both sides.
Mix the wet ingredients: In a large bowl, whisk together the mashed bananas, melted butter, granulated sugar, and brown sugar until smooth and well combined. Add the eggs one at a time, whisking after each addition, then stir in the vanilla extract and sour cream or Greek yogurt.
300 grams very ripe bananas, 113 grams unsalted butter, 100 grams granulated sugar, 80 grams brown sugar, 2 large eggs, 2 teaspoons pure vanilla extract, 60 grams sour cream or full-fat Greek yogurt
Add the dry ingredients: Add the flour, cocoa powder, baking soda, and fine sea salt directly to the bowl of wet ingredients. Fold gently with a spatula until just combined, stopping as soon as no dry streaks remain. The cocoa powder makes it especially easy to overmix, so keep the folding light and gentle. Cocoa powder makes overmixing even easier to do accidentally, so be gentle.
200 grams all-purpose flour, 25 grams Cocoa powder, 1 teaspoons baking soda, 0.5 teaspoons fine sea salt, ½ teaspoon cinnamon
Fold in the chocolate: Reserve about 30g of the chocolate chunks for the top. Fold the remaining 200 grams of chocolate, roughly chopped into chunks, into the batter until evenly distributed.
200 grams dark chocolate
Fill the pan and top: Pour the batter into the prepared pan and smooth the top. Press the reserved chocolate chunks over the surface, then finish with a generous pinch of 1 teaspoons flaky sea salt (for topping).
1 teaspoons flaky sea salt
Bake: Bake for 60–70 minutes, until a toothpick inserted into the center comes out with just a few moist crumbs or until the internal temperature is 200F. The chocolate chunks on top will look melty and glossy — that's perfect. Tent loosely with foil after 45 minutes if the top is darkening too quickly, but I find that I never need to do this.
Cool before slicing: Cool in the pan for 15 minutes, then lift onto a wire rack using the parchment overhang. Wait at least 45 minutes before slicing — the chocolate chunks need time to set back up or every slice will fall apart.