Preheat the oven to 350 degrees F. Butter a 9x5-inch loaf pan; set aside.
Combine the brown sugar, cocoa, flour, granulated sugar, baking soda, baking powder, and salt in the bowl of an electric mixer. Mix on low speed just to combine.
In a separate bowl, whisk together the eggs and egg yolk to blend, then add the buttermilk, oil and vanilla; whisk until combined.
Turn the mixer to low and slowly pour the wet ingredients into the dry ingredients, mixing just until combined. Stir in the chopped chocolate by hand using a rubber spatula. Give it all a good last stir, making sure to scrap up everything on the bottom and sides and mixing it in.
Pour the batter into the prepared pan and bake for 1 hour to 1 hour 10 minutes, or until a toothpick inserted in the center comes out clean. Let the loaf cool in the pan for 15 minutes, then turn it out onto a wire rack to cool completely.
To Make The Cream Cheese Spread:
Using a spoon, mix the cream cheese in a medium bowl until soft and fluffy. Add the peanut butter and stir to completely combine, then add the sugar and stir briskly, until incorporated.
Store the loaf wrapped tightly with plastic wrap at room temperature; store the cream cheese spread covered with plastic wrap (or in an airtight container) in the refrigerator
From the kitchen Brown Eyed Baker. Recipe adapted from Baked Explorations.