Place an oven rack in lower-middle position and preheat the oven to 350F. Line an 8-inch square baking dish with foil and spray lightly with nonstick cooking spray; set aside.

In a heatproof bowl set over a pan of barely simmering water, melt the chocolate and butter, stirring occasionally until smooth. Whisk in the cocoa powder until smooth. Set aside to cool.

In a medium bowl, combine the eggs, sugar, vanilla and salt; whisk until combined, about 15 seconds. Whisk in the warm chocolate mixture until incorporated. Then stir in the flour with a wooden spoon until just combined. Fold in walnuts. Pour the mixture into the prepared pan and spread with a spatula to make an even layer. Bake until slightly puffed and a toothpick inserted in the center comes out with a small amount of sticky crumbs clinging to it, 35-40 minutes. Transfer the pan to a wire rack and let cool to room temperature before cutting, about 2 hours.