Swiss Meringue Buttercream

5 large egg whites
1 1/4 cup sugar
1 pound (4 sticks) butter, cut into tablespoons, room temperature..I used salted butter, just a personal preference
2 teaspoons pure vanilla extract (I used vanilla bean paste)

Combine egg whites and sugar into the heatproof bowl of a standing mixer set over a pan of simmering water, much like a double boiler. Whisk constantly by hand until the mixture is sugar has dissolved (the mixture should feel completely smooth when rubbed between your fingertips, no grainy sugar), approx 2 minutes.
Put mixing bowl on mixer and using the whisk attachment start mixing.  Begin on low, raising the speed up to medium high. Beat until stiff peaks form, about 10 minutes.
Now add your butter, 2 tablespoons at a time.  Beating on medium until each addition of butter is added.
Now add your vanilla.
Remove the whisk attachment and replace it with your paddle attachment.
Mix with paddle for 2 minutes on low speed working out the air bubbles.
Makes enough frosting to cover and fill 1 8" layer cake.
Store airtight in fridge, for, according to Martha, 3 days.