Heat the oven to 350°F (175°C) and line baking sheets with parchment paper. If using toasted walnuts, toast them at 350°F for 7–10 minutes, cool completely, and then chop them coarsely.

Whisk the flour, cocoa powder, baking soda, and salt together in a medium bowl. Mixing the dry ingredients first distributes the cocoa and leavening evenly throughout the dough.

Beat the softened butter, granulated sugar, and packed brown sugar for about 2–3 minutes, until the mixture looks light and creamy. Add the eggs one at a time, beating after each addition, then mix in the vanilla.

Add the dry ingredients in two or three additions, mixing only until no dry streaks remain. Fold in the semisweet chocolate chips or chunks and the chopped walnuts without overmixing.

Scoop approximately 2 tablespoons of dough for each cookie and arrange the portions 2 inches apart on the prepared baking sheets. For thicker, chewier cookies, chill the scooped dough for 30–60 minutes before baking.

Bake for 10–12 minutes, until the edges look set but the centers still appear slightly soft. Cool the cookies on the baking sheet for 5–10 minutes, then transfer them to a wire rack to finish setting.