Preparation: First, heat your oven to the temperature recommended on the cake mix box, typically around 350 degrees Fahrenheit (165 degrees Celsius). Prepare a 9-inch round cake pan by greasing it with cooking spray or butter and lightly flouring it.
Melt Chocolate: Next, melt your bittersweet or semisweet chocolate. You can do this by placing it in a heatproof bowl over a pot of simmering water until it's nice and smooth. Set it aside to cool for a bit.
Mix Wet Ingredients: Now, in a big mixing bowl, combine the chocolate cake mix, brownie mix, and cocoa powder. Then, add in the oil, eggs, egg yolk, buttermilk (or regular milk), salt, and vanilla extract. Mix everything together until it's smooth and well blended.
Incorporate Melted Chocolate: Slowly pour in the melted chocolate while stirring continuously until it's all mixed in. This will give your cake a rich chocolate flavor and make it extra moist.
Fold in Chopped Chocolate: Carefully fold in the chopped bittersweet or semisweet chocolate pieces until they're evenly distributed throughout the batter. These chocolate chunks will give your cake a delicious texture.
Bake: Place the round cake pan in the preheated oven and bake for about 35-40 minutes, or until a toothpick inserted into the center of the cake comes out clean.
Cool: Once it's done baking, take the cake out of the oven and let it cool in the pan for about 10 minutes. Then, carefully flip it onto a wire rack to cool completely before frosting.