Preheat the oven to 325°. Spray a standard 9 x 5 inch loaf pan with baking spray.
In the bowl of a stand mixer with the paddle attachment, beat the butter until creamy. Add the brown and granulated sugars and beat until light and fluffy, about 3 minutes. Scrape down the sides of the bowl and beat in the egg until completely combined. Beat in the vanilla and the chocolate chips until evenly distributed.
Sift the flour, cocoa powder, baking soda, and salt together into a small bowl. Really do take the time to sift, as cocoa powder tends to clump. Add about 1/3 of the flour mixture and 1/3 of the buttermilk. Repeat with remaining buttermilk and flour. Mix until ingredients are all incorporated, scraping down the sides of the bowl, as necessary.
Scrape the batter into the prepared pan and bake for 60 to 70 minutes until a toothpick inserted in the center comes out clean. Cool in the pan on a rack for about 10 minutes, then remove the cake from the pan and place on a wire rack to cool completely.
Place a piece of parchment or waxed paper under the rack where the cake has cooled to catch any drips.
Heat the buttermilk in a small saucepan over medium heat just until it begins to bubble. Do not boil. The buttermilk will begin to separate; that’s fine. Add the chocolate and turn off the heat. Beat vigorously with a spatula or spoon until the chocolate is melted and smooth. Beat in the cubes of butter until melted and smooth. Because the buttermilk curdles slightly, this ganache is not utterly smooth and silky, but I like it that way — it gives it a homemade look. If you like, you can pour the ganache through a fine strainer into a bowl before spreading it on the cake.
Slowly spread the ganache over the top of the cake. I love to leave most of the ganache on the top, with just a little overflow dripping down. Go slowly and you can do this too. This way you don’t lose too much frosting in drips. Let the ganache cool and firm up for at least an hour, then slice and serve