Center a rack in the oven and preheat the oven to 350 degrees F. Line a 9X13-inch pan with foil and lightly grease and flour (I like to use Baker’s Joy to make the greasing/flouring part easier).
For the congo layer: in a small bowl, stir together the flour, baking powder, and salt. In a large bowl, stir together the butter, brown sugar, egg, egg white, and vanilla just until blended. Stir in the flour mixture just until the batter is smooth.
Scoop the thick, sticky batter onto 8 or 9 different areas on the bottom of the prepared pan. With an offset spatula or rubber spatula, spread the batter to cover the pan as evenly as possible. Sprinkle the walnuts evenly on top and gently press them into the batter.
Partially bake the layer until it is no longer shiny on top and is beginning to come away from the sides of the pan, about 12-15 minutes (it will continue baking with the brownie layer). Transfer the pan to a wire rack to cool while you prepare the brownie layer.
For the brownie layer: in a small or medium bowl, stir together the flour, cocoa powder, baking powder, and salt. In a small saucepan, melt the butter and chocolate over very low heat, stirring with a silicone spatula until smooth. Or, combine in a medium microwave-safe bowl and melt in a microwave oven at 50 percent power (medium) for 30-second bursts, stirring after each burst, until melted, 1 to 1½ minutes. Transfer to a medium bowl and let cool for 5 minutes.
Stir the sugar into the chocolate mixture until incorporated. Add the eggs, egg yolk, and vanilla and stir until well blended. Stir in the flour mixture just until the batter is smooth. Scoop the thick, sticky batter onto 8 or 9 different areas of the still-warm congo layer. With the offset spatula, spread the batter as evenly as possible.
Bake the brownie until the top is no longer shiny; appears set; feels firm, rather than soft; and a round wooden toothpick inserted in the center comes out with moist chocolate crumbs attached, about 20 minutes. Transfer the pan to a wire rack and let cool completely. The top layer will set up as it cools.
To make the ganache drizzle: place the chocolate in a medium bowl. In a small saucepan, bring the cream just to a boil over medium heat. Pour the cream over the chocolate and let it stand for 30 seconds. Whisk together until smooth and creamy. Set the mixture aside at room temperature just until it cools a bit and begins to thicken slightly.
Pour the warm glaze into a small ziploc bag. Snip a small opening at one of the corners to allow a thin flow of glaze. Squeezing lightly, pipe thin lines of chocolate in a zigzag pattern over the cooled surface of the brownie. Set aside at room temperature until the glaze is set, 1 to 1½ hours.