Preheat oven to 375° Fahrenheit. Line the bottom of a baking pan (13 x 9 x 2 in) with parchment paper; set aside.
Roughly chop the chocolate. Add both butter and chocolate to a bowl and melt in microwave. Mix together.
Whip the eggs and sugar on high speed for 5 minutes in a stand mixer or with a hand mixer.
Fold the creamed egg and sugar mixture into the melted chocolate with a rubber spatula, incorporate well.
Sift the flour, cocoa powder and salt through a sifter or a fine sieve into the bowl of wet ingredients. Mix for 30/40 seconds or until incorporated; scraping down the sides of the bowl with a rubber spatula.
This is the time to add anything you like - nuts, white chocolate or fruits. Just fold it in. Pour into baking pan.
Bake on 180 degrees Celsius or 375 degrees Fahrenheit for 15 minutes.