Prepare the Pan:

1. Preheat your oven to 350°F (175°C).
2. Line a 9-by-13-inch glass baking dish with aluminum foil, leaving a 2-inch overhang on each end for easy removal.

Make the Brownie Batter:

1. In a small bowl, whisk together the flour, cocoa powder, baking powder, and salt.
2. In a saucepan, bring the butter, brown sugar, and 1/2 cup water to a simmer. Remove from heat and whisk in the chopped unsweetened chocolate until smooth. Let cool slightly.
3. Whisk in the eggs and vanilla extract until well combined.
4. Stir in the dry ingredients until just combined. Set aside.

Make the Banana Bread Batter:

1. In a medium bowl, whisk together the flour, granulated sugar, baking soda, cinnamon, and salt.
2. In a large bowl, beat the egg. Add the mashed bananas, sour cream, melted butter, and vanilla extract, mixing until well combined.
3. Stir in the dry ingredients until just combined.

Assemble and Bake:

1. Use a medium ice cream scoop to dollop about half of the banana bread batter into the prepared baking dish.
2. Spoon or pour the brownie batter around the banana bread batter, partially covering it.
3. Dollop the remaining banana bread batter over the top, ensuring some brownie batter remains visible.
4. Bake for 35 to 40 minutes, or until a toothpick inserted into the center comes out clean.
5. Allow the brownies to cool in the baking dish on a rack for about 1 hour.

Prepare the Topping:

1. Lift the brownies out of the pan using the foil overhang, but leave them in the foil.
2. In a small heatproof bowl, combine the butter and unsweetened chocolate. Microwave in 30-second intervals, stirring between each, until melted and smooth.
3. Sift the confectioners’ sugar into a medium bowl.
4. Whisk in the melted chocolate mixture and boiling water to create a smooth glaze, adding more water if necessary to achieve a drizzling consistency.

Glaze and Serve:

1. Drizzle the glaze over the cooled brownies.
2. Sprinkle the toasted walnuts evenly over the top.
3. Let the glaze set for about 30 minutes before cutting into squares and serving.