- Prep the oven and cupcake liners. Heat oven to 350°F (180°C). Line two cupcake pans with about 16 liners total (or you can bake multiple batches with one pan).
- Mix together the dry ingredients. In a large mixing bowl or the bowl of a stand mixer, whisk together cake flour, sugar, baking powder, baking soda and salt until combined.
- Add the butter. Add the butter (be sure that it is room temperature) and mix on medium-low speed for 3 minutes, scraping down the sides as needed. The batter should have a fine crumb texture.
- Whisk together the wet ingredients. In a separate small bowl, whisk together the eggs, sour cream, oil and vanilla extract until smooth.
- Add the wet ingredients. Add the egg mixture to the flour mixture and beat on medium-speed for 30 seconds, or until just combined. Reduce speed to low and gradually add the milk until it is just combined. The batter will be liquid.
- Bake. Fill the cupcake liners so that they are just over halfway full. Bake for 14 to 16 minutes, or until a toothpick inserted in the middle comes out clean. Transfer the pan to a wire rack and let the cupcakes cool completely to room temperature.
- Beat the butter. Add the butter to a large mixing bowl or the bowl of a stand mixer and beat on medium speed for 2 to 3 minutes until it is light and fluffy.
- Add the remaining ingredients. Add the powdered sugar, heavy cream and vanilla extract and beat on low speed for 1 minute until combined. Scrape down the sides of the bowl, then beat on medium speed for 2 more minutes until completely smooth.
- Adjust consistency if needed. If the buttercream seems too thin, add in 1 to 2 cups of additional powdered sugar and beat until smooth. If the buttercream seems too thick, add in 1 tablespoon of heavy cream (or more, if needed) and beat until smooth.
- Decorate the cup cake and enjoy
