Begin by preparing your pan. Line the 12x17" rimmed baking sheet with parchment paper and spray generously with nonstick cooking spray or oil. Set aside. Next prepare the melted chocolate mixture - in a small pot over simmering water, place a medium sized bowl.Add in the 3.75 cups of milk chocolate chips, the 2 cups of unsalted butter, and the 1/4 cup of cocoa powder. Mix well with a rubber spatula, and continue to mix until melted.
Once melted, remove from heat and set aside. Let cool for 10-15 minutes.
Once slightly cooled, preheat the oven to 350°F.
Next, in a large bowl, add all the wet ingredients (the eggs, sugar, and vanilla extract).Mix until just incorporated (don't beat or over mix). Add the cooled chocolate mixture into this wet mixture and mix until well incorporated.
In a separate medium-sized bowl, sift then mix the flour, baking powder, and salt.
Once combined, add to the chocolate and wet mixture in the large bowl.
Mix until just incorporated.
In a separate small bowl, mix the semisweet chocolate chips and the chopped walnuts. Sprinkle about 1/8 teaspoon of salt onto them and about 1/8 cup of flour.Toss them well with the flour. This will help them from sinking to the bottom of the brownies. Once mixed, fold the walnut and chocolate chip mixture into the brownie batter.
Once incorporated, pour the batter into the prepared baking sheet.