Preheat oven to 350°F (175°C) and line a baking sheet with parchment paper.

Mix dry ingredients: In a bowl, whisk flour, cocoa powder, baking soda, and salt.

Cream butter and sugars: In another bowl, beat butter, brown sugar, and granulated sugar until light and fluffy.

Add egg and vanilla: Mix in the egg and vanilla extract until combined.

Combine wet and dry ingredients: Gradually add the dry ingredients to the wet mixture.

Fold in chocolate chips and walnuts.

Scoop dough onto the baking sheet, spacing them apart.

Bake for 10-12 minutes until edges are set but centers remain soft.

Cool on the baking sheet for 5 minutes, then transfer to a wire rack.