Place the butter and sugar in the bowl of a stand mixer. Beat on medium speed until fluffy, about 3–4 minutes.
Turn off the mixer and add the egg. Beat until combined.
In a medium bowl, combine the cocoa, flour, baking soda, and salt. Whisk to combine.
With the mixer on low, add half of the buttermilk. Once it is incorporated, add half of the flour mixture. Repeat with the remaining buttermilk and flour mixture.
Add the chopped pecans and mix until just combined. Refrigerate the dough for at least 2 hours, or up to 48 hours.
Preheat the oven to 375°. Use a spoon to scoop out one tablespoon of dough and place on a cookie sheet lined with parchment paper or a silicone mat. Repeat with the remaining dough, leaving 3 inches of space between each tablespoon of dough (the cookies will spread).
Bake for 8–10 minutes, until the cookies no longer look wet on top. Let sit for 5 minutes on the cookie sheet. Remove the cookies and place on a cooling rack.