Silky smooth Swiss Meringue Buttercream perfect for cakes, cupcakes and other tasty desserts!
Place the egg whites and sugar in the bowl of stand mixer and set over a pot of gently simmering water. Stir with a spatula until the sugar has dissolved and is warmed to the touch, about 160°F. When you touch it with your fingers, it should feel completely smooth and not gritty.
Immediately transfer bowl to a stand mixer fitted with a whisk attachment. On high speed, whip until glossy and stiff, about 15 minutes.
Lower the speed to medium. Add the cut butter, piece by piece, incorporating each before adding more. Lower the speed and mix until silky and smooth. (It will look runny while adding butter and then it will come back together, nice & fluffy.)
It will look like fresh whipped butter when completed, add flavoring or color at this point. Place the mixing bowl into the refrigerator to set up.
After 30 mins, put the chilled buttercream into a pipping bag and use to decorate cupcakes, cookies and cakes.
To make ahead: Keep in airtight container in refrigerator for up to one week or in the freezer for up to 2 months. Let come to room temperature and re-whip in the mixer with the paddle attachment before using.
To use under fondant: Frost the cake as smooth as possible. Place in refrigerator until the buttercream has hardened before covering in fondant. Cover with fondant straight from the refrigerator.