When an Italian Meringue is made the sugar syrup cooks the whites and heats them to 160 degrees, which is hot enough to kill any micro-organisms that could be in the egg whites. You really don’t need to be worried about the safety of the whites.
A Swiss meringue buttercream is different from the Italian in that the whites, sugar, salt, and acid (cream of tarter or lemon juice) are whisked together over a water bath until the it reaches a temp no higher than 165 degrees. Some recipes call for heating it to 140-145 (which will dissolve the sugar, but not make it completely safe to eat). You will have a more stable Swiss Meringue Buttercream if you heat the mixture to at least 160 before whipping.
There is a textural difference between the two styles of buttercream. The Italian Meringue style is by far the most stable of buttercreams (it is least likely to slide off a cake and melt in warmer temperatures), lighter in texture, somewhat easier to work with when icing a cake, and has a fluffier texture in the mouth and melts easily on the tongue. The Italian Meringue is the best for layer cakes, especially tiered cakes.
The Swiss Meringue in contrast is slightly heavier in texture (although not heavy like ganache is), and if the egg whites are not heated to 165 before whipping, it is not stable enough to use in temperatures 90 degrees and above. It also does not have the fluffy mouse like texture that Italian Meringue Buttercreams have, and consequently if you use it for a layer cake, you have to periodically re-beat it by hand a few times so it spreads easily.
Given it’s few disadvantages, I still use the SMB for simple layer cakes, as it is simple to make and takes less time put together. It is also my favorite choice for cupcakes, and can be flavored with just about anything (a few tablespoons of jam beat into the buttercream is delightful on vanilla cupcakes).
Here is my version of Swiss Meringue Buttercream. It is somewhat lighter than the standard recipe (1/2 cup egg whites to 1 cup of sugar and 1# butter), so it works better for layer cakes than the standard version (although it does not spread AS easily as the IMB).
3/4 cup of egg whites (6-8 egg whites, depending on the size of the egg white)
1 cup + 1/4 cup plain white sugar
3/4 tsp table salt
1 tablespoon lemon juice or 3/4 tsp cream of tarter (I prefer lemon juice for the flavor)
1# of unsalted butter, cut into smallish cubes and allowed to come to room temp.
2 tsp of extract of your choice
1-2 Tablespoon of flavored liquor of your choice
Make sure the mixing bowl, whisk, and whip are spotlessly clean.
In the bowl of a standing mixer, whisk the whites, sugar, salt, and acid together. Place the mixing bowl over a pot of barely simmering water and heat until it reaches 160-165 degrees on an instant read thermometer. Whisk constantly while it’s heating to prevent the whites from cooking on the bottom of the bowl.
Put the bowl on the mixer with the whip attachment and start the machine on medium speed for about 1 minute. When the whites begin to thicken, increase the speed to high and whip until the whites have reached full volume, are fluffy, and form stiff, glossy peaks (FYI, it’s almost impossible to over whip these egg whites because of the sugar, acid, and pre-cooking them).
Allow the whites to whip until they cool completely and are under 100 degrees (the bottom of the bowl should feel barely warm to the touch).
Decrease the speed to medium, and gradually add in the butter about a tablespoon at a time, whip in each addition of butter into the meringue thoroughly before adding more butter. Periodically increase the speed to high, briefly, to make sure butter is emulsifying properly. The butter cream may break and curdle when you get to the last 1/3 of the butter; this is completely normal, just continue whipping until it comes back together.
When the last of the butter is added and the buttercream is well emulsified, add the extract and liquor and whip until well blended (it may break again briefly, but will come back together quickly).