Preparation

Make the brownies

• Position a rack in the center of the oven and heat the oven to 350°F. Line a 9-inch-square metal baking pan with foil, leaving an overhang on two sides for easy removal of the brownies. Butter and flour the bottom and sides of the foil, tapping out the excess flour.

  Melt the butter in a 3-quart saucepan over medium heat until it smells nutty and turns golden, 4 to 5 minutes. Remove the pan from the heat and let cool for 5 minutes. Whisk in the sugar and salt, followed by the eggs, banana, and vanilla. Whisk in the cocoa powder and flour, mixing slowly at first and then more vigorously until the batter is combined.

  Spread the batter in the prepared baking pan, smoothing it so it fills the pan evenly. Bake until a toothpick or a skewer inserted in the center of the pan comes out with just a few moist clumps clinging to it, 40 to 45 minutes. Let the brownies cool in the pan before topping.

Make the topping

• While the brownies cool, bring the cream to a boil in a small saucepan over medium-high heat. Remove from the heat. Stir the chopped banana into the cream; let the mixture steep for 1 hour.

  Put the chopped chocolate in a medium heatproof bowl. Bring the cream to a boil over medium-high heat, stirring occasionally. Pour the cream mixture through a strainer held directly over the bowl of chopped chocolate. Discard the banana. Let the chocolate mixture stand for 1 minute; then stir until smooth. Pour the ganache evenly over the cooled brownies.

  Position a rack 6 inches from the broiler and heat the broiler on high. Cover the ganache with the marshmallows and almonds. Broil, rotating the pan every 20 seconds or so to keep the marshmallows from burning, until browned. Using a knife, free the marshmallow topping from the sides of the pan. Let the brownies cool in the pan until the ganache is set, at least 1-1/2 hours. Using the foil overhang, remove the brownies from the pan and cut into 16 squares (use a wet knife to keep the marshmallows from sticking).