Heat the oven to 350°F. Cover your cookie sheets with parchment. In a medium bowl, mix the flour, cocoa powder, baking soda, and salt. With an electric mixer, beat the butter and both sugars on medium-high speed until light and fluffy, scraping down the sides of the bowl as needed. Beat in the eggs, one at a time, and then the vanilla, scraping down the sides of the bowl. Add the flour mixture and mix on medium low just until combined. Scrape down the sides of the bowl and stir in the chocolate and pecans.
Drop the dough by the heaping tablespoonful about 2 inches apart onto the cookie sheets. Bake until they’re still very soft but no longer look wet, about 8 minutes. (These cookies are especially good and chewy if under baked at this short time; otherwise the cookie is overly crisp and hard when cooled.) Let cool on the sheets for 5 minutes; transfer to a wire rack to cool completely.