Preheat your oven to 350°F (175°C) for even baking.
Line a 9x13 inch baking pan with parchment paper, leaving overhang for easy removal.
Melt the unsalted butter, semisweet chocolate, and unsweetened chocolate together in a double boiler or microwave-safe bowl until smooth.
In a large mixing bowl, whisk together the granulated sugar and eggs until well combined.
Stir in the vanilla extract followed by the cooled chocolate mixture until fully incorporated.
In a separate bowl, whisk together the flour, baking powder, and salt, then fold into the chocolate mixture until just combined.
Fold in the chopped walnuts and chocolate chips.
Pour the batter into the prepared baking pan and bake for 25-30 minutes until a toothpick inserted comes out with moist crumbs.
Allow to cool in the pan for 15 minutes, then lift out using the parchment and cool completely before cutting.
