Line an 8x8 inch baking pan with foil, leaving a two inch over hang on both sides.
Mix flour and cocoa powder in a small bowl. Set aside.
To the melted butter, add sugar, salt and vanilla. Mix briefly.
Add eggs and mix vigorously until just combined, about 15-20 times total.
Add the flour and cocoa mixture and fold in until smooth. Fold in nuts and chopped chocolate-if using.
Pour the batter into the baking dish and spread evenly, smoothing top, pushing batter to the edges.
****Bake @ 350°F for 30-35 minutes until edges become stiff but center is still soft and moist (but not uncooked) to the touch. Toothpick inserted in the middle should not be clean but have little crumbs. [30 minutes was adequate in my oven]
Lift out of pan using foil overhang and place on a wire rack to cool. Cut into 16 (or 9) pieces.
Notes
****Wrap leftovers in plastic and store in the refrigerator.