Place egg whites and powdered sugar in a stand mixer bowl. Attach the whisk and combine ingredients on low and then whip on high for 5 minutes
Add in your butter in chunks and whip with the whisk attachment to combine. It will look curdled at first. This is normal. It will also look pretty yellow. Keep whipping.
Let whip on high for 8-10 minutes until it's very white, light and shiny.
Switch to a paddle attachment and mix on low for 15-20 minutes to make the buttercream very smooth and remove air bubbles. This isn't required but if you want really creamy frosting, you don't want to skip it.
Serving: 2g | Calories: 849kcal (42%) | Carbohydrates: 75g (25%) | Protein: 2g (4%) | Fat: 61g (94%) | Saturated Fat: 38g (190%) | Cholesterol: 162mg (54%) | Sodium: 240mg (10%) | Potassium: 18mg (1%) | Sugar: 74g (82%) | Vitamin A: 2055IU (41%) | Calcium: 18mg (2%) | Iron: 0.4mg (2%)