Preheat the oven to 350 degrees F. Spray a 9x13-inch pan with nonstick spray and then line it with parchment paper, allowing enough overhand on the long side to lift the brownies from the pan.
Place the butter and chocolates in a small saucepan and cook, stirring, over the lowest possible heat until melted, 3 to 5 minutes. Set aside to cool.
In a large mixing bowl, use a whisk to combine the sugar, eggs and vanilla, Add the cooled chocolate mixture and whisk until just combined.
In a separate bowl, stir together the flour, baking powder and salt. Stir into the wet ingredients until there are no white streaks. Scrape the batter into the prepared pan and sprinkle with the nuts and the chocolate chips.
Transfer to the oven and bake until your kitchen smells like chocolate and a toothpick inserted into the center comes out clean, 30 to 35 minutes. Set aside to cool to room temperature, then cover and refrigerate. When the brownies are fully cooled, cut into 24 squares.