I'd like to make a true buttercream frosting. I usually make Rose's Mousseline Buttercream, and would prefer to base this frosting off of it.
The basic method is to whip a few egg whites to soft peaks, then stream in 248 degree sugar syrup, whip til cool, then add in a pound of softened butter a tablespoon at a time, then flavor. I'd prefer not to add all of the sugar syrup and then add more sweetness in the form of dulce de leche. Would I deflate the egg whites by adding the dulce in? Experience with this?