For the brownies: Preheat the oven to 350 degrees F. Line a 9-by-13-inch glass baking dish with aluminum foil so that 2 inches hang over each end.
Whisk the flour, cocoa powder, baking powder and salt together in a small bowl. Bring the butter, brown sugar and 1/2 cup water to a simmer in a saucepan, then turn off the heat. Whisk in the unsweetened chocolate, then let cool slightly. Whisk in the eggs and vanilla. Stir in the flour mixture. Set aside while you make the banana bread.
For the banana bread: Whisk the flour, granulated sugar, baking soda, cinnamon and salt together in a medium bowl. Beat the egg in a large bowl. Add the bananas to the egg and mix to combine well. Stir in the sour cream, melted butter and vanilla. Stir in the flour mixture until just combined.
Use a medium ice cream scoop to dollop about half of the banana bread batter in the prepared baking dish. Spoon or pour the brownie batter around the banana bread batter (it will cover it partially). Scoop the remaining banana bread batter over the top, making sure some brownie batter still shows through. Bake until a toothpick inserted in the center comes out clean, 35 to 40 minutes. Cool in the baking dish on a rack, about 1 hour.
Lift the brownies out of the pan using the foil overhang, but leave in the foil.
For the topping: Combine the butter and chocolate in a small heatproof glass bowl. Microwave in 30-second intervals, stirring in between each, until melted. Sift the confectioners' sugar into a medium bowl. Whisk in the melted chocolate and the boiling water to make a smooth glaze, adding a little more water if it is too thick to drizzle. Drizzle the glaze over the top of the brownies with a spoon. Sprinkle the walnuts all over. Let the glaze set, about 30 minutes. Cut into squares and serve.
When measuring flour, we spoon it into a dry measuring cup and level off excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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