Preheat the oven to 350°F (175°C). Position a rack in the center. Butter a 9 x 5-inch loaf pan, dust it with flour, and tap out the excess flour.

Put the 3/4 cup firmly packed dark brown sugar into the bowl of a stand mixer fitted with the paddle attachment and break up any large lumps with the back of a spoon. Add the 1 cup sifted dark unsweetened cocoa powder, 1 1/2 cups all-purpose flour, 3/4 cup granulated sugar, 1 1/2 teaspoons baking soda, 3/4 teaspoon baking powder, and 1 teaspoon salt.

Mix the dry ingredients on low just until they are evenly combined, then scrape down the sides and bottom of the bowl so everything is incorporated.

In a separate bowl, whisk together the 2 large eggs and 1 large egg yolk until blended. Add the 3/4 cup buttermilk, 1/2 cup vegetable oil, and 1 teaspoon vanilla extract; whisk until smooth.

With the mixer on low, slowly pour the wet mixture into the dry ingredients in a steady stream, mixing only until the batter is just combined and there are no large dry streaks. Do not overmix.

Fold in the 8 ounces coarsely chopped dark chocolate by hand with a spatula until the chocolate is evenly distributed.

Pour the batter into the prepared loaf pan and smooth the top with a spatula. Tap the pan once or twice on the counter to release any large air bubbles.

Bake until a toothpick inserted into the center comes out clean or with a few moist crumbs, about 1 hour to 1 hour 10 minutes. If the top is browning too quickly, tent loosely with foil for the remaining time.

Remove the pan from the oven and let the cake cool in the pan on a wire rack for 15 minutes. Run a knife around the edges, invert the loaf onto the rack, and remove the pan. Cool completely before slicing.

Slice and serve with whipped cream and strawberries, if desired.