Preheat oven to 350°F. 

Lightly grease a square 8x8 baking dish. Line greased pan with parchment paper, leaving overhang on all four sides. Use binder clips to secure parchment paper to edges if necessary. 

In a large bowl, melt butter with 6oz chopped semi-sweet chocolate. Stir well to combine and ensure no chunks remain. 

Slowly add sugar, whisking until fully encorporated. Add one egg at a time, whisking well between additions. Add in vanilla, salt and cocoa powder, whisking until smooth. Add flour, folding until no streaks remain. Stir in chocolate chips or nuts. Pour into prepared baking sheet. Level off with an offset spatula. 

Bake for 40-45 minutes or until a toothpick inserted in the center comes out clean. 

Cool in pan at least 15-30 minutes before carefully slicing into equal portions. 

Store remaining brownies in an airtight container for up to a week.