Preheat the oven to 350 F / 180 C.
In a saucepan, melt the butter.
Add the cocoa powder to the saucepan and stir until completely incorporated.
Take the saucepan off the heat and allow it to sit for several minutes.
Add 2 eggs and the egg yolk to a bowl with 1 cup of white sugar. Using an electric mixer, beat on medium speed for 4-5 minutes, until the mixture is pale and thickened.
Add the flour and salt to the egg mixture and fold with a spatula.
Melt the dark chocolate chips in a new saucepan or in a bowl in the microwave.
Add the melted dark chocolate chips and begin to fold them into the egg mixture.
Add the melted butter and cocoa powder to the bowl. Fold until both the melted chocolate and butter are completely incorporated.
Add in the chocolate chips and walnuts and fold one more time.
Pour the batter into a greased 8x8 metal baking dish.
Bake at 350 F / 180 C for 30 minutes.
Let the brownies rest at room temperature for one hour.
Enjoy!