Preheat the oven to 350˚F and line 2 baking sheets with parchment paper. For the cookies: Melt the chocolate chips in a microwave-safe bowl, stirring at 50 percent power in 15-second intervals until smooth. Let it cool slightly.
In a medium bowl, whisk together flour, cocoa powder, baking powder, baking soda, and salt.
In a large bowl, beat together butter, granulated sugar, and brown sugar until smooth and light using a mixer on medium-high speed (about 2 minutes). Add the egg, vanilla, and melted chocolate, beating until well combined.
With the mixer on low speed, beat in half of the flour mixture, then add the buttermilk, and finally, the remaining flour mixture until smooth. Mix in the pecans.
Drop heaping tablespoonfuls of dough on the baking sheets about 2 inches apart. Bake for 8 to 10 minutes, rotating the pans halfway through. Ensure the centers remain fudgy; avoid overbaking. Let them cool for 5 minutes on the baking sheets before transferring to a rack set over a parchment-lined baking sheet.
For the glaze: Melt the butter in a small saucepan. Remove from heat and whisk in milk, cocoa powder, and vanilla until smooth. Add powdered sugar and whisk vigorously until smooth. Spoon the glaze over the cookies, allowing it to drip down the sides. Let the glaze set for about 1 hour.