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1. Preheat your oven to 175°C (350°F). Grease and line a large rimmed sheet pan (roughly 33x23 cm / 13x9 inch) with parchment paper.
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2. In a large bowl, mash the very ripe bananas thoroughly until almost completely smooth — the riper and blacker the bananas, the more flavor and sweetness they bring.
4 piece Very ripe bananas
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3. Melt the butter and whisk it into the mashed bananas along with both sugars until well combined. Add the eggs one at a time, whisking after each, then stir in the vanilla extract and sour cream until smooth.
225 g Unsalted butter,
300 g Granulated sugar,
100 g Brown sugar,
3 piece Large eggs,
2 tsp Vanilla extract,
240 g Sour cream
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4. In a separate bowl, sift together the flour, cocoa powder, baking soda, baking powder, and salt. Gradually fold the dry ingredients into the wet mixture using a spatula, mixing until just combined — do not overmix.
280 g All-purpose flour,
75 g Unsweetened cocoa powder,
1.5 tsp Baking soda,
1 tsp Baking powder,
1 tsp Salt
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5. Stir in the hot coffee (or hot water) until the batter is smooth and glossy — this blooms the cocoa and deepens the chocolate flavor. Fold in half the chocolate chips.
120 ml Hot coffee or hot water,
100 g Dark chocolate chips
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6. Pour the batter into the prepared sheet pan and spread it evenly to the edges. Bake for 28–33 minutes, or until a toothpick inserted in the center comes out with only a few moist crumbs. Allow the cake to cool completely in the pan before frosting.
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7. Heat the heavy cream in a small saucepan over medium heat until just simmering. Pour it over the remaining dark chocolate chips in a heatproof bowl and let sit for 2 minutes, then stir until silky smooth. Allow this ganache to cool for 10 minutes.
100 g Dark chocolate chips,
180 ml Heavy cream
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8. In a separate bowl, beat the softened frosting butter with the powdered sugar, cocoa powder (for frosting), and milk until fluffy and smooth. Pour in the cooled ganache and beat again until the frosting is glossy and spreadable. If it's too thick, add a splash more milk; if too thin, chill for 5 minutes.
150 g Powdered sugar,
40 g Cocoa powder (for frosting),
3 tbsp Milk,
60 g Butter (for frosting)
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9. Spread the chocolate frosting generously and evenly over the completely cooled cake using an offset spatula or the back of a spoon. Allow the frosting to set for at least 15 minutes before slicing into squares and serving directly from the pan.
