Step 1: Preheat your oven to 350°F (177°C/gas mark 4) and place the baking rack in middle.
Step 2: Fill a saucepan with 1 to 2 inches (2.54 to 5.08 centimeters) of water and place a heat safe bowl on top; make sure that the bottom of the bowl does not touch the surface of the water. Add the chopped chocolate and the butter, and turn the heat to medium. Stir the butter and chocolate with a whisk until they melt.
Step 3: Set the melted chocolate aside. Once the chocolate and butter are melted, mixed together, and smooth, take the bowl off the saucepan and set it aside. Allow it to cool to lukewarm temperature before continuing.
Step 4:Stir in the sugar and vanilla extract, then add stir in the eggs, one at a time.
Step 5: Whisk in the salt and flour. If you used salted butter, reduce the salt to just ½ teaspoon. Keep whisking until the flour is evenly mixed in and there are no lumps of clumps.
Step 6: Add the chocolate chips, and if desired, the chopped walnuts as well.
Step 7: Pour the batter into a lightly greased and floured 9 by 13-inch (22.86 by 33.02 centimeters) baking pan.
Step 8: Bake the brownies for 25 to 30 minutes, then let them cool for 20 minutes.
Step 9: Prepare the frosting, if desired.
Step 10: Pour the frosting over the brownies and enjoy your delicious treat!