Preheat the oven to 350F degrees. Line the bottom and sides of a 9x13 inch pan with parchment paper, or line with aluminum foil and lightly grease.
Add the butter to a medium saucepan and melt over low-medium heat.
Once the butter is melted, whisk in the cocoa powder. Remove from the heat.
In a separate bowl beat the eggs and sugar for at least 4 minutes, or until they become pale in color and double in volume.
Beat the eggs into the melted butter & cocoa powder.
Mix in the vanilla extract.
Using a rubber spatula or wooden spoon, gently stir in the salt and flour.
Optionally stir in the chocolate chips or chopped nuts.
Pour the batter into the prepared pan and bake for 30-35 minutes, or until the top looks set and an inserted toothpick comes out clean or with a few damp crumbs.