Prepare your ingredients, you'll need egg whites, granulated sugar and butter, and vanilla and a little salt for flavor.
Place the granulated sugar with the egg whites in a heatproof bowl and cook in a double boiler until 131F°/55°C.
Whip the egg-white and sugar mixture for about 20-30 minutes until you obtain a glossy Swiss meringue with stiff peaks.
Add progressively the butter pieces, vanilla and salt and mix until you have a smooth, creamy frosting.
Use the Swiss meringue buttercream frosting immediately to fill and decorate your cakes and desserts.