Sarah's Italian Meringue Buttercream
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.
If syrup looks crystallized in step 2, it is because it was heated too quickly and you will need to start the syrup over. If buttercream looks curdled in step 4, continue to beat until smooth.
Vanilla Bean: Beat in seeds scraped from 1 vanilla bean pod
Chocolate: Beat in 1/2 cup semisweet chocolate that has been melted and cooled
Raspberry or strawberry: Beat in 1/4 to 1/2 cup raspberry or strawberry puree
Other flavors: Add 1 to 2 teaspoons flavor extract
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