01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, leaving overhang on two sides for easy removal.
02 - In a heatproof bowl set over a saucepan of simmering water, melt the butter and chopped chocolate together, stirring until completely smooth. Remove from heat and let cool slightly for about 5 minutes.
03 - Add both the granulated sugar and light brown sugar to the chocolate mixture. Whisk until well combined and smooth.
04 - Add eggs one at a time, whisking thoroughly after each addition until fully incorporated. Stir in the vanilla extract.
05 - Sift the flour, cocoa powder, and salt together directly into the batter. Gently fold the dry ingredients into the wet mixture using a spatula until just combined. Do not overmix.
06 - If using, fold in the chopped nuts and/or dark chocolate chips until evenly distributed throughout the batter.
07 - Pour the batter into the prepared baking pan and smooth the top evenly with a spatula.
08 - Bake for 22 to 25 minutes, or until the top is set and the edges pull away slightly from the pan. The center should still have a slight jiggle for a fudgy texture.
09 - Allow the brownies to cool completely in the pan on a wire rack before slicing into 12 squares.