"This is sort of a finicky frosting, but so good. Once you master this type of buttercream, you will never go back to the overly sweet powdered sugar type icing. It's light and fluffy and just the right sweetness. It's the type of frosting found in most high-end gourmet pastry shops. You will need a candy thermometer for this recipe. Makes enough to generously frost a two-layer 9-inch round cake."
Pour egg whites into the clean bowl of a stand mixer fitted with a whisk attachment; beat at medium speed until frothy. Gradually add 1/4 cup sugar, continuing to beat until stiff peaks form.
Combine remaining 1 cup sugar and water in a small saucepan. Attach a candy thermometer to the saucepan and place over low heat. Cook until syrup reaches 244 degrees F (117 degrees C) or until a small amount of syrup dropped into cold water forms a firm but pliable ball.
Increase mixer speed to high and pour syrup in a slow and steady stream down the side of the bowl, beating until combined, 1 to 2 minutes. Reduce speed to medium-high and continue beating until meringue is stiff and the sides of the bowl feel barely warm, 3 to 4 minutes more.
Beat in butter 1 tablespoon at a time until smooth. Switch to the paddle attachment, add vanilla extract, and beat on high until buttercream is light and fluffy, 4 to 7 minutes.