Preheat oven to 400°F.
If eggs are not room temperature, add whole eggs into a bowl with warm water and let sit for 5-10 minutes. Crack eggs into a separate bowl, and set aside. Doing this gives you a chance to fish out any unwanted egg shells!
In a microwave-safe bowl, microwave the butter at 15-30 second intervals until completely melted. Set aside until ready to incorporate into the recipe.
Measure sour cream and milk into a small, microwave-safe bowl. Microwave for 30 seconds to bring to room temperature and set aside.
Add the all purpose flour, granulated sugar, box cake mix, and salt to a large mixing bowl. Use a whisk or spoon to gently combine the ingredients until they are evenly distributed.
Add all other ingredients including milk, sour cream, melted butter, and eggs 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 sticking to the bottom of the bowl. Mix until batter is smooth and fluffy, usually 1-2 minutes!
Set batter aside to rest for 15 minutes.
Prepare the cupcake pan by lining with cupcake liners while the batter is resting.
Fill each cupcake liner 2/3 full with cake batter.
Bake at 400°F for 5 minutes, then lower oven temperature to 350°F for 7-8 minutes. Use a toothpick insert into the center of a cupcake – 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 one of the cupcakes, if it springs back slowly, it’s done!
Cool in pan for 5 minutes, and transfer cupcakes to a wire cooling rack to finish cooling.