Preheat oven to 350°F and lightly spray one 9x13 inch OR two 8x8 inch pans with non-stick spray.
In a heavy bottomed sauce pan, double boiler, or microwave, melt the butter and unsweetened chocolate together until just melted. Set aside to cool slightly.
In a large bowl, add the melted butter and chocolate, and whisk until combined.
Add the sugars and vanilla and whisk vigorously until all of the sugar is combined.
Add the eggs, one at a time, whisking to make sure the whole egg is evenly incorporated before adding the next.
Sift the cocoa powder into the mixture using a sift or a fine mesh sieve. (You can skip sifting it, however it helps to relieve your brownies of cocoa clumps which are very unpleasant to bite into.
In a separate, medium sized bowl, add the flour and the salt. Mix them thoroughly.
Add the flour to the rest of the ingredients. The mixture will be very thick and you might have to switch from a whisk to a spatula to finish incorporating the flour.
Fold in the chocolate chips or shavings if you are using those.
Pour batter into prepared pan(s). If you are baking in a 9x13 inch pan, bake it for about 35-45 minutes, and if you are baking in two 8x8 inch pans, bake for 25-30 minutes.
Allow to cool for 15 minutes before cutting and eating.