Preheat the oven to 350 degrees. Butter and flour a 9x5 inch loaf pan.
Place the brown sugar in the bowl of a mixer fitted with the paddle atttachment. Press out any lumps with the back of a spoon. Add the cocoa, flour, granulated sugar, baking soda, baking powder, and salt and mix until combined.
In a seperate bowl, whisk the eggs and egg yoolk 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, mixing until just combined. Stir in the dark chocolate chunks by hand. Do not over mix!
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 comes out clean.
Cool in the pan for 10 minutes and then turn out onto a cooling rack.
For the peanut butter spread:
In the bowl of a stand mixer, beat together all ingredients until well combined and fluffy. Use immediately or store tightly wrapped in the refrigerator for up to 3 days.