3 1/2 cups (793g)unsalted butter, softened and cut into cubes
1/4tspfine sea salt
1 1/2tbsppure vanilla extract
1tbsppure vanilla bean paste
Weigh out egg whites and granulated sugar together in a non-reactive bowl, clip a candy thermometer onto the side of the bowl and place over a pot of simmering (not boiling) water, whisking constantly and gently until temperature reaches 160F.
Using a sieve, strain the egg white sugar mixture into the bowl of a stand mixer.
Mix on medium speed with the whisk attachment until the mixture is glossy, reaches stiff peaks and the outside of the bowl is no longer warm. You cannot add butter to the bowl if it is warm or the butter will just melt when added.
Once the mixture reaches stiff peaks, you have Swiss Meringue (Yay!) but we’re going for buttercream here, so onward. Switch out the whisk attachment for the paddle attachment and begin mixing on low speed.
Add the softened butter cubes, a couple at a time, until incorporated.
Now, just let it mix. It might curdle or look lumpy but that’s ok. Keep mixing. This could take some time. The buttercream is ready when it is smooth, satiny and creamy.
Keep mixing on low and add the salt and flavorings. Mix until everything is fully incorporated.
This recipe can be halved or doubled. One full recipe will fill and frost a 8" cake that is 4" or 5" tall.