Preheat oven to 350° F and prepare your pans. For cake pans, spray with nonstick cooking spray and line with parchment circles. For cupcakes, line a cupcake pan with cupcake liners.
In a large mixing bowl, add chocolate cake mix, all-purpose flour, granulated sugar, dark cocoa powder, milk, melted butter, vegetable oil, sour cream, eggs, and vanilla extract.
Using an electric hand mixer, mix on low speed until the ingredients come together.
Once the ingredients are combined, turn the speed on the mixer to medium and mix for a full 2 minutes.
Portion the cake batter into your cake pans or cupcake liners.
Baking times will vary depending on the size of your pans or if you are making cupcakes. For cupcakes, baking time is approximately 20 minutes. For 6-inch or 7-inch cakes, baking time is approximately 30 minutes and for 8-inch or 9-inch cakes, baking time is approximately 35 minutes. When the cakes are perfectly baked, they will bounce back when gently touched or an inserted toothpick will come out clean.
Remove the pans from the oven. Allow cakes to cool in the pan for about 15 minutes and cupcakes to cool for 5 minutes.
Remove the cake/cupcakes from the pan and transfer to a wire rack to cool completely.