01 - Set the 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 over simmering water, melt butter and chopped chocolate together, stirring until smooth. Remove from heat and let cool slightly.
03 - Whisk in granulated sugar. Beat in eggs one at a time, followed by vanilla extract.
04 - Sift flour, cocoa powder, and sea salt together; gently fold into the chocolate mixture until just combined.
05 - Fold in walnuts and/or chocolate chips if using.
06 - Spread the batter evenly into the lined baking pan.
07 - Bake for 25 to 30 minutes, until a toothpick inserted in the center emerges with a few moist crumbs.
08 - Allow brownies to cool completely in the pan before lifting out and slicing into 16 squares.