Ever since I discovered Swiss meringue buttercream, I have never wanted any other buttercreams for my cakes. The texture is light and creamy and is smooth enough that it looks great piped as decoration. And it freezes well too. All you need to do is to thaw it in the fridge then re-whipped it for a minute on a stand mixer and you are good to go.
Instead of specific weight, you use proportions for this, so, you can measure out sugar and butter as per the weight of available egg whites.
1 portion egg whites
1 portion granulated sugar
2 portions unsalted butter, at room temperature
1) Set up a bain-marie and whisk granulated sugar with egg whites in a bowl set atop the bain-marie. Keep whisking until sugar is completely melted. To test, just pinch the mixture and make sure it does not feel grainy.
2) Transfer the egg mixture into the bowl of your stand mixer and whisk on medium-high speed for 10 minutes. By then, the bowl of the mixer should have cooled and the egg mixture would have at least double in volume.
3) Turn down mixer to medium speed and add room temperature butter, a tablespoon at a time, letting it incorporate before adding next tablespoon.
As I use this recipe as a base and add other flavours to the buttercream (chocolate, caramel, etc.) I normally use less sugar than the original 1.5x. It works perfectly fine with granulated sugar amounting anywhere between 1 to 1.5 times the weight of the egg whites.