Preheat your oven to 350°F (175°C).
In a medium saucepan over low heat, combine the coarsely chopped semi-sweet chocolate, unsweetened chocolate, and unsalted butter. Stir frequently until melted and smooth. Remove from heat and let cool slightly.
In a large mixing bowl, whisk together the granulated sugar and eggs until thick and pale, about 2-3 minutes.
Stir in the vanilla extract and the slightly cooled chocolate mixture until well combined.
In another bowl, whisk together the all-purpose flour, baking powder, and kosher salt.
Gently fold the dry ingredients into the chocolate mixture, being careful not to overmix.
Fold in the coarsely chopped walnuts and semi-sweet chocolate chips until evenly distributed.
Line a 9x13 inch baking pan with parchment paper, pour in the brownie batter, and spread it evenly.
Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
Let the brownies cool in the pan on a wire rack for at least 15 minutes, then lift out using the parchment overhang and cool completely before cutting into squares.