Melt butter over medium heat or slowly in the microwave. Remove from heat and whisk in cocoa powder. Scrape into medium-sized bowl with a spatula. Add sugar and salt, whisking to combine until smooth. Add eggs, one at a time, mixing after each addition until blended, adding vanilla extract with the last egg. Using a large spoon or spatula, mix in flour just until blended. Pour into prepared pan, and gently spread evenly over the bottom of the pan. Set aside.