Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper.
In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.
Add the eggs, one at a time, mixing well after each addition. Stir in the vanilla extract until combined.
In another bowl, whisk together the flour, cocoa powder, baking soda, and salt.
Gradually add the dry ingredients to the wet mixture, alternating with the buttermilk, starting and ending with the dry ingredients. Mix until just combined.
Fold in the semi-sweet chocolate chips and chopped pecans if using.
Using a tablespoon or cookie scoop, drop cookie dough balls onto the prepared baking sheets, leaving about 2 inches of space between each.
Bake in the preheated oven for 10-12 minutes, or until the edges are set but the centers are still soft.
Remove from the oven and let the cookies cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely.
Notes
Arrange the cookies on a large platter and dust lightly with powdered sugar for an elegant touch.