Place the egg whites and powdered sugar in the bowl of a stand mixer. Using the whisk attachment, combine the ingredients on low speed and then whip on high speed for 1 to 2 minutes to dissolve the powdered sugar.
Add in the salt and vanilla extract.
Add the softened butter in chunks and whip with the whisk attachment for 8 to 10 minutes until it no longer tastes like butter. It will look curdled at first but this is normal, just keep whipping.
If your buttercream continues to look curdled, remove about 1/3 cup of buttercream and melt it in the microwave for 10-15 seconds until it is barely melted. Pour it back into the whipping buttercream to bring it all together.
Switch to the paddle attachment and mix on low for about 10 minutes to remove the air bubbles and create super smooth buttercream. This is optional but will make it extra creamy.
Add about 1 cup (8 oz) of room temperature strawberry puree and lemon zest to the buttercream and mix until combined. You can add additional strawberry puree a quarter cup at a time until your desired flavor is reached. The more puree you add the less structure the buttercream will have.
This buttercream has fresh fruit in it, so make sure it stays refrigerated. This recipe makes enough frosting to cover and fill an 8-inch layer cake. Store excess buttercream in the fridge for 2 weeks or freeze it for up to 6 months. Defrost the buttercream in the fridge and re-whip it before using.