Begin by preheating your oven to 350°F (175°C). Line a baking sheet with parchment paper to prevent sticking and ensure even baking.
In a medium-sized bowl, whisk together the all-purpose flour, cocoa powder, baking soda, and salt. Set this mixture aside as it will be combined with the wet ingredients later.
In a large mixing bowl, combine the melted butter and packed brown sugar. Stir vigorously until the mixture is smooth and well blended, ensuring there are no lumps.
Add the large egg and vanilla extract to the butter-sugar mixture. Mix until the ingredients are fully incorporated and the mixture is glossy.
Gradually add the dry ingredient mixture to the wet ingredients, folding it in gently until just combined. Be cautious not to overmix, as this can affect the texture of the cookies.
Gently fold in the dark chocolate chips and chopped walnuts (if using) until they are evenly distributed within the cookie dough.
Using a spoon or a cookie scoop, drop rounded spoonfuls of dough onto the prepared baking sheet, ensuring they are spaced about 2 inches apart to allow for spreading.
For extra indulgence, press a few additional dark chocolate chips into the tops of the cookie dough balls before baking.
Bake in your preheated oven for 10-12 minutes, or until the edges are set and the centers are still slightly soft to the touch. Keep an eye on them to avoid overbaking.
Allow the cookies to rest on the baking sheet for about 5 minutes after removing from the oven. Then, transfer them to a wire rack to cool completely.
Notes
Serve warm with milk or ice cream for an extra treat.