This recipe is one of the most decadent (retro) desserts we found in a tiny Brooklyn bakery.
Place brown sugar in the bowl of an electric mixer fitted with a paddle attachment. Press out any lumps with the back of a large spoon. Add cocoa, flour, granulated sugar, baking soda, baking powder, and salt.
In a separate bowl, whisk egg and egg yolks until blended, then add buttermilk, vegetable oil, and vanilla; whisk until combined.
Turn mixer to low and pour in egg-buttermilk mixture in a thin steady stream, mixing just until combined. Remove the bowl; stir in chocolate chunks by hand with a wooden spoon.
Pour the batter into the prepared pan and bake in the center of oven for 1 hour, or until a toothpick inserted in the center of loaf comes out clean.
Let cool in pan for 15 minutes; remove and place on rack to cool completely. Serve plain or toasted; top with whipped cream (optional).