Preheat the oven to 350°F (175°C). Grease a loaf pan and line it with parchment paper, leaving a little overhang so the loaf lifts out easily later.
In a large bowl, whisk together the flour, sugar, cocoa powder, baking soda, and salt until evenly mixed and free of lumps.
In another bowl, whisk the buttermilk, vegetable oil, eggs, and vanilla extract until smooth.
Pour the wet mixture into the dry ingredients and stir gently just until combined. Stop as soon as the flour disappears to keep the loaf tender.
Fold in the chopped pecans and dark chocolate chips with a spatula. The batter should be thick and glossy.
Transfer the batter to the prepared pan and smooth the top. Tap the pan lightly on the counter to release any trapped air.
Bake for 55 to 60 minutes, until a toothpick inserted in the center comes out clean or with a few moist crumbs. If the top browns too quickly, cover it loosely with foil during the last 10 minutes.
Let the loaf cool in the pan for 10 minutes, then remove and transfer to a wire rack. Allow it to cool completely.
Heat the heavy cream in a small saucepan over medium heat until it just starts to simmer. Remove from heat and pour it over the chopped dark chocolate. Let it sit for 5 minutes, then stir until smooth and shiny.
Drizzle the ganache over the cooled loaf and let it set for about 15 minutes before slicing.