1. Preheat the oven to 350 degrees F and position the rack in the center. Butter a 9 by 5 inch loaf pan, dust it with flour, and knock out the excess flour.
2. Place the brown sugar in the bowl of a standing mixer fitted with the paddle attachment. Press out any lumps with the back of a large spoon. Add the cocoa, flour, granulated sugar, baking soda, baking powder, and salt. Scrape down the sides and bottom of the bowl.
3. In a separate bowl, whisk the eggs and egg yolk until blended, then add the buttermilk,oil, and vanilla; whisk until combined. Turn the mixer to low and slowly stream the wet ingredients into the dry ones, mixing just until combined. Stir in the dark chocolate chunks by hand.
4. Pour the batter into the prepared pan and bake for 1 hour to 1 hour and 10 minutes, or until a toothpick inserted in the center of the loaf comes out clean. Let the cake cool in the pan for 15 minutes, then turn it out onto a wire rack to cool completely.
5. Slice the cake and serve with whipped cream and strawberries, if desired.