Preheat oven to 345°F. Prepare (4) 6 inch cake pans or (3) 8 inch cake pans with baking spray or a homemade pan release. You can also use parchment paper rounds at the bottom of the pans for extra insurance.
In a microwave-safe bowl, microwave the butter at 15-30 second intervals until completely melted. Set aside until ready to incorporate into your recipe.
Measure sour cream and milk into a small, microwave-safe bowl. Microwave for 30 seconds to bring to room temperature and set aside.
If eggs are not room temperature, add whole eggs into a bowl with warm water and let sit for 5 minutes. Crack eggs into a separate bowl, and set aside. Doing this gives you a chance to fish out any unwanted egg shells!
Add the flour, sugar, cake mix, cocoa powder, and salt to a large mixing bowl. Use a whisk or spoon to gently combine the ingredients until they are evenly distributed.
Combine all ingredients (minus the chocolate chips) into the bowl containing the dry ingredients. Don't forget to add the vegetable oil and vanilla extract!
Mix ingredients together with an electric hand mixer, starting on low and increasing speed to med-high. Scraping bowl periodically to make sure all ingredients are incorporating and not stuck on the bottom. Mix until batter is smooth and fluffy, usually 1-2 minutes!
Pour batter into prepared cake pans. Use a food scale to evenly distribute batter between pans for consistent cake layer size.
(Optional) Using a fork, gently press a tablespoon of chocolate chips per pan evenly into batter. After gently pressing into the batter, move batter around with the fork to cover any exposed chocolate chips. This keeps the chocolate chips from burning!
Bake for 30-32 min for 6 inch pans, and 28-30 min for 8 inch pans. Use a toothpick or knife and insert into the center of the cake – if it comes out clean or with a few moist crumbs, it’s good to go. You can also gently press your finger into the top of the cake, if it springs back slowly, it's done!
Cool in pan for 10-15 minutes, and transfer to a wire cooling rack to finish cooling.