01 - Preheat oven to 350°F. Line an 8x8-inch square baking pan with parchment paper, leaving an overhang for easy removal.
02 - In a heatproof bowl set over simmering water, melt butter and chopped chocolate, stirring until smooth. Remove from heat and allow to cool slightly.
03 - Whisk in sugar, then beat in eggs one at a time. Stir in vanilla extract.
04 - Sift flour, cocoa powder, and salt together. Fold into chocolate mixture until just combined.
05 - Gently fold in chopped nuts and/or chocolate chips, if using.
06 - Spread batter evenly into prepared pan.
07 - Bake for 25 to 30 minutes, until a toothpick inserted in the center comes out with a few moist crumbs.
08 - Cool completely in pan before lifting out using parchment overhang. Cut into 16 squares to serve.