Preheat oven to 350 degrees F and place the oven rack in the middle section.
Line a 8x8x2 inch pan with tin foil. Spray foil with cooking spray.
Add cocoa and sugar. Stir until well blended.
Beat eggs and mix with vanilla.
Add to the cocoa mixture and blend.
Gradually add flour and salt. Whisk well.
Fold in walnuts and transfer into the prepared pan and bake for 30 minutes, until a toothpick inserted in the center comes out clean.
Cool and slice before serving.