Prepare the Batter: In a microwavable bowl (or on the stovetop), combine the white sugar, brown sugar, and butter. Heat until bubbly, stirring occasionally. Remove from heat and beat well with a hand mixer until fully combined.
Add the eggs and vanilla extract to the sugar-butter mixture. Beat for two minutes with a hand mixer. This step, along with melting the butter and sugar, helps create a crackly top on the cookies.
Incorporate Dry Ingredients: Stir together the cocoa powder and salt, then add them to the batter. Beat for another 30 seconds until well mixed.
Add Mix-ins: Fold in the dark chocolate chips and chopped walnuts (if using). The batter might be thin, but it will thicken as it rests.
Let the batter rest while preheating the oven to 350°F (175°C). Lightly grease a cookie sheet or line it with parchment paper.
Scoop and Bake: Once the batter is scoopable but still sticky, use about 2 tablespoons of batter per cookie, placing scoops on top of each other for a stacked effect. Bake the cookies for 12-15 minutes.
Finishing Touch: After removing the cookies from the oven, press a few extra chocolate chips on top for an appealing finish.