Place egg whites and powdered sugar in a mixing bowl with the whisk attachment. Whisk to combine. Add in butter in small chunks then vanilla and salt. Whip on high until light and fluffy and white. Optional: switch to the paddle attachment and mix on low for 15-20 minutes until all air bubbles are gone.
Combine 2 cups room temperature puree and zest with buttercream. Fold together to combine.
Because this buttercream has fresh fruit in it make sure it stays refrigerated. Store excess buttercream in the fridge or freeze for up to 6 months. Defrost buttercream in the fridge and re-whip before use.
Calories: 1707kcal (85%) | Carbohydrates: 154g (51%) | Protein: 5g (10%) | Fat: 122g (188%) | Saturated Fat: 77g (385%) | Cholesterol: 325mg (108%) | Sodium: 287mg (12%) | Potassium: 95mg (3%) | Sugar: 149g (166%) | Vitamin A: 4105IU (82%) | Vitamin C: 23.2mg (28%) | Calcium: 42mg (4%) | Iron: 0.9mg (5%)