In a large bowl, sift together the flour, cocoa, and baking soda. Set aside.

Use a stand mixer with the paddle attachment. In the mixer's large bowl, beat the butter, white sugar, and brown sugar together on medium speed until fluffy and smooth. Beat in the salt and pure vanilla. Slowly beat in the flour mixture, a 1/2 cup at a time, until the dough is chucky and moist and sticks together.

Stir in the chocolate chunks and pecans. Turn the dough out onto a lightly floured counter and knead it until it forms a dough ball. Divide the dough in half and shape each half into a lot that is 1 1/2 inches in diameter. Length doesn't matter--just make sure no part of the log is hollow. Wrap each log in plastic wrap and refrigerate for at least 3 hours.

When ready to bake, preheat the oven to 325℉. Then adjust the rack so it sits in the center of the oven. Line 2 baking sheets with parchment paper.

Work with one log at a time, keeping the other in the fridge while you bake the first log. On a lightly floured counter or cutting board, use a very sharp knife to slice the dough into 1/2" rounds. If the rounds crack, just mush them back together when you place them on the baking tray. Place all the circles on the baking sheets, about 2" apart.

Bake the cookies for 12 minutes. They will still look soft and mushy, but they are done and will firm up as they cool. Let the baking sheet cool on a metal baking rack and let the cookies set for 10 minutes. Then move the cookies to another baking rack to cool completely.

Repeat baking cookies with the second log until you've used up all the dough.