Preheat oven to 350°F (175°C). Grease an 8-inch square baking pan. If you prefer, line the pan and grease the lining and any exposed parts of the inside of the pan.
Whisk together the flour, cocoa powder, and salt. Set aside.
Stir together the butter, sugar, and brown sugar. Add the eggs, one at a time, mixing well after each addition. Then, mix in the vanilla.
Add the flour mixture to the butter mixture, and mix just until combined. Stir in the nuts, chocolate chips, and white chocolate chips.
Transfer the batter to the prepared pan and spread evenly. Bake 22-25 minutes, or until a toothpick inserted into the center comes out with moist crumbs.
Cool completely in the pan before cutting into bars.