Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper—this helps cookies release cleanly.

In a medium bowl, whisk together flour, cocoa powder, baking powder, baking soda, and salt. Set aside. (Pro tip: sift the cocoa for clump-free dough.)

In a large bowl, cream butter with both sugars until light and fluffy—about 3 minutes with a hand mixer on medium speed.

Beat in the egg and vanilla until smooth, scraping down the sides. Then stir in the buttermilk until just combined.

Gradually add the dry ingredients to the wet, mixing on low speed until you see a uniform, slightly sticky dough. Fold in chocolate chips by hand.

Using a 1½-Tbsp cookie scoop (or a heaping tablespoon), drop dough 2 inches apart on prepared sheets. Bake 9–11 minutes until centers look set but still soft. They’ll firm as they cool—resist the urge to overbake.

Place chopped chocolate in a heatproof bowl. Warm the cream and butter in a small saucepan over medium heat until bubbles form at the edges, then pour over chocolate. Let sit 1 minute, then whisk until silky.

As soon as the cookies come out, spoon a tablespoon of ganache on each warm cookie and spread gently. Sprinkle chopped pecans right away so they stick.

Let cookies rest on the sheet for 10 minutes, then transfer to a wire rack to cool completely.