Prepare the Brownies:
Preheat and Prepare: Set the oven to 325°F. Line the bottom of an 8 x 8-inch square baking pan with parchment paper or foil, allowing an overhang on two sides for easy removal. Grease the pan thoroughly, including the foil if used.
Melt the Butter: In a medium saucepan over medium-low heat, melt the butter, swirling occasionally to ensure even melting.
Combine Ingredients: Remove from heat and whisk in the cocoa powder until smooth. Stir in the sugar and salt—the mixture will be dense and grainy, which is expected.
Incorporate Eggs and Vanilla: Vigorously whisk in the eggs, one at a time, until the mixture turns silky and smooth. Stir in the vanilla extract.
Add Flour: Introduce the flour all at once, switching to a spatula to mix. Stir with force for about 50 strokes until a uniform batter forms.
Bake: Transfer the batter to the prepared pan, smoothing the top. Bake for approximately 25 minutes or until a toothpick inserted in the center emerges clean. Allow the brownies to cool completely on a wire rack, then cover with foil and refrigerate for at least 4 hours.
Prepare Brownie Chunks:
Cut the Brownies: Lift the chilled brownies from the pan using the parchment or foil flaps. Slice in half, reserving one portion for later consumption.
Dice the Brownies: Cut half of the brownies into 1/2-inch cubes, yielding about 290g (2 to 2 1/2 cups) of chunks.
Prepare the Banana Bread:
Preheat the Oven: Heat the oven to 350°F and grease an 8 1/2 x 4 1/2 or 9 x 5-inch loaf pan.
Melt the Butter: In a small saucepan, gently melt the butter over low heat, then set aside.
Mix the Batter: In a large bowl, mash the bananas with a whisk until mostly smooth. Add sugar and whisk vigorously to help break down the bananas further.
Incorporate Wet Ingredients: Whisk in the egg, vanilla extract, and salt, followed by the melted butter.
Combine Dry Ingredients: Using a spatula, gently fold in the flour and baking soda until just combined. If the batter appears excessively thick, incorporate a few tablespoons of water to loosen it.
Integrate the Brownie Chunks: Carefully fold in the diced brownie pieces, distributing them evenly throughout the batter. Pour the mixture into the prepared loaf pan.
Bake:
Bake the Loaf: Transfer to the oven and bake for about 1 hour or until the center springs back when lightly pressed.
Cool the Bread: Let the bread rest in the pan for 10 minutes before turning it out onto a cooling rack to cool completely. For precise slicing, refrigerate the loaf before cutting. While delicious chilled, serving at room temperature allows the flavors to fully develop.