Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9×9 inch baking pan. Sift the cocoa powder so that it's free of lumps.
In a sauce pan, brown the butter until it gives off a nutty fragrance. Remove from heat and stir in 4 oz of the semi sweet chocolate and stir until melted.
2 oz butter, 4 oz semi sweet chocolate chips
In a separate bowl, combine the sweetened condensed milk, sugar, eggs, and vanilla extract. Beat mixture until completely blended.
14 oz sweetened condensed milk, ¼ cup sugar, 2 large eggs, 1½ teaspoon vanilla extract
Pour in the brown butter and melted chocolate mixture and mix together until combined.
Fold in the sifted cocoa powder, flour, and salt until combined with a spatula.
½ cup flour, ½ cup cocoa powder, ¼ teaspoon salt
Mix in the remaining 4 oz semi sweet chocolate morsels, or chopped chocolate with the spatula.
4 oz semi sweet chocolate chips
Pour the brownie batter into the prepared baking pan. Sprinkle the top with some additional chocolate morsels or chopped chocolate if desired.
Bake in the preheated oven for 40 to 45 minutes, checking with a toothpick at 40 minutes.
Remove from oven and allow to cool for 30 minutes before cutting into squares or rectangles.