Preheat oven to 350 degrees F. Line a 9x13 inch metal pan with foil and grease bottom only.
In a saucepan or in a microwave-safe bowl, melt the butter. Add the chopped chocolate to the hot melted butter and stir until fully melted. If using the microwave, you may have to microwave it a bit more, stirring every 15 seconds, until melted and smooth. Set aside to cool slightly.
In a mixing bowl, combine the salt (if using), brown sugar and vanilla. Add the melted chocolate mixture and stir by hand (if using a stand mixer, use the paddle attachment on low) until blended. Gradually add the eggs, stirring until blended. Lastly, stir in the flour until fully blended.
Pour mixture into prepared pan. If desired, sprinkle chocolate chips or chunks over the top. Bake for 25-30 minutes (though check at 20) or until brownies appear set. Let cool at room temperature or quick cool by putting the 9x13 inch pan in a larger roasting pan and filling 2/3 up the sides with ice water.
When brownies are completely cool, lift from pan and cut into 24 bars.