01 - Preheat the oven to 350°F. Line a 9-inch square baking pan with parchment paper, leaving an overhang on two sides for easy removal.
02 - In a heatproof bowl, melt the chocolate and butter together over a pot of simmering water (double boiler), stirring until smooth. Remove from heat and let cool slightly.
03 - Whisk the granulated and brown sugars into the melted chocolate mixture until well combined.
04 - Add the eggs one at a time, whisking thoroughly after each addition until the batter is glossy and thick.
05 - Sift together the flour, cocoa powder, baking powder, and salt. Gently fold the dry ingredients into the chocolate mixture until just incorporated—do not overmix.
06 - Fold in chopped nuts or chocolate chips if using.
07 - Pour the batter into the prepared pan and smooth the top evenly with a spatula.
08 - Bake for 28 to 32 minutes, or until the center is just set and a toothpick inserted comes out with a few moist crumbs.
09 - Allow the brownies to cool completely in the pan on a wire rack before lifting out and cutting into squares.