1. Preheat & Prep: 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.
2. Melt Butter: In a microwave-safe bowl, microwave the butter at 15-30 second intervals until completely melted. Set aside.
3. Wet Ingredients: Measure sour cream and milk into a small, microwave-safe bowl. If not already at room temperature, microwave for 30 seconds to bring to room temperature and set aside.
4. Eggs: If eggs are not room temperature, add whole eggs (with shell) 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!
5. Dry Ingredients: Add flour, sugar, yellow cake mix, and salt to a large mixing bowl. Use a whisk or spoon to gently combine the dry ingredients.
6. One Bowl: Combine all ingredients into the larger bowl containing the dry ingredients. Don’t forget to add the vegetable oil and vanilla extract (or imitation vanilla)!
7. Mix It Up! Whisk or mix ingredients together with an electric hand mixer

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   , 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!
8. Pour into Pans: Pour batter into prepared cake pans. If you can, use a food scale

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   to evenly distribute batter between pans for consistent cake layer size.
9. Bake: Bake for 30-32 min for 6 inch pans, and 28-30 min for 8 inch pans. Insert a toothpick or knife 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!
10. Cooling: Let cakes cool in pan for 10-15 minutes, and transfer to a wire cooling rack to finish cooling.
11. Storage: Double wrap your cake layers in plastic wrap and freeze for up to 3 months. When you’re ready to decorate, simply take out and frost straight from frozen—no need to thaw!

For the Chocolate Buttercream

1. Whip the Butter: Start by adding the room temperature butter to a stand mixer, fitted with the paddle attachment. Mix the butter on medium-high for 30 seconds-1 minute, scraping the sides of the bowl at least once during this time.
2. Vanilla & Salt: Add the vanilla extract and salt. 
3. Start Heavy Cream: Add 2 tablespoons of heavy cream and continue mixing until incorporated. Scrape the bowl.
4. Powdered Sugar: Lower mixer speed to the lowest setting, and start adding the powdered sugar in 1-2 cup increments. Add a tablespoon of heavy cream or milk between each.
5. Mix It Up! Slightly increase the mixer speed to help blend these ingredients, but do not turn on too high or you will whip too much air into the buttercream.
6. Reminder: Scrape the sides and bottom of the mixing bowl to make sure all of ingredients are incorporated and to avoid any clumps of butter or powdered sugar. 
7. Cocoa Powder: After all ingredients are combined, finish by adding the unsweetened cocoa powder.
8. Last Step: Mix on low-medium until all ingredients are incorporated, this could take a few minutes. At this point you should have perfectly smooth, chocolate buttercream!