Italian buttercream uses hot sugar syrup poured into whipped egg whites, creating an exceptionally stable and luxurious frosting. Heat sugar and water to 240°F (soft ball stage) while simultaneously whipping egg whites to soft peaks. Slowly pour the hot syrup into the whites while beating continuously. Continue whipping until the bowl is cool to touch, then gradually beat in softened butter. The result is incredibly smooth, less sweet than American buttercream, and holds its shape beautifully for piping.