Buttercream is wonderfully versatile, and many kinds of flavorings can be added. This recipe is for strawberry IMBC, but strawberries can be omitted or replaced.
In a medium saucepan, bring the water and sugar to a soft boil of 245° F. While your sugar is cooking, in a separate mixing bowl, lightly whisk egg whites and salt until soft peaks form (a stand mixer is ideal, but a hand mixer will work).
Once sugar syrup has reached temperature, crank mixer up on high and slowly pour the sugar syrup into eggs. Allow meringue to come to stiff peaks, and make sure bowl is cool to touch. This may take 10 minutes or more.
Meringue must be stiff enough to stay put when scooped to the side of the bowl. If it is at all runny, keep mixing. Change to a paddle attachment and add butter. (If your butter is firm enough to pick up between your fingers, it is too hard. Leave it out to soften overnight for best results.) Mix on the lowest setting. It will look curdled and soupy. Do not mix on high. This step can take up to 15 minutes.
When your buttercream is fluffy, light and smooth, it is done. Add pureed strawberries. Your frosting may appear curdled or lumpy at first, but keep mixing until smooth again, approximately 7 minutes.