• Butter , for greasing pan
• 3/4 cup tightly-packed dark brown sugar
• 1 cup Dutch cocoa powder , sifted
• 1 1/2 cups all-purpose flour , plus more for dusting
• 3/4 cup granulated sugar
• 1 1/2 tsp. baking soda
• 3/4 tsp. baking powder
• 1 tsp. salt
• 2 eggs
• 1 egg yolk
• 3/4 cup buttermilk
• 1/2 cup vegetable oil
• 1 tsp. vanilla
• 8 ounces semisweet (60 percent) chocolate chunks
• Whipped cream (optional)

Preheat oven to 350°. Butter one 9" x 5" x 3" loaf pan, dust with flour, and knock out excess flour.

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).