01 - Preheat the oven to 350°F. Line a 9 x 9 inch baking pan with parchment paper, leaving an overhang for easy removal.
02 - In a medium bowl, whisk together the flour, cocoa powder, baking powder, and salt until evenly mixed.
03 - In a large bowl, whisk the melted butter with granulated and brown sugars until smooth and glossy.
04 - Add eggs one at a time to the butter and sugar mixture, beating well after each addition, then stir in the vanilla extract.
05 - Gradually fold the dry ingredients into the wet mixture until just combined without overmixing.
06 - Fold in the semisweet chocolate chips and roughly chopped mixed nuts evenly.
07 - Pour the batter into the prepared pan and spread it evenly with a spatula.
08 - Bake for 28 to 32 minutes or until a toothpick inserted in the center comes out with a few moist crumbs.
09 - Cool completely in the pan on a wire rack before lifting out and cutting into 12 squares.