Okay, I would really like to try to make IMBC but I am so confused because there are so many differant directions on how to make it. I have been searching for recipes and they all say differant things....
The butter should be...
Room temp?
Chilled?
Softened?

Heat the sugar water to...
248 degrees?
235 degrees?
238 degrees?

Use the whisk or paddle attachment?
I have found so many recipies and I do not think that one is the same. What to do??
TIA

Can anyone help me??

I am only familiar with the recipe I use. The higher the heat the less chance of bacteria in the egg whites. But if you cook it too long or at too high a temperature you will have lumps in the sugar syrup that will transfer to the icing. I have heard people say they strain the syrup into another container before adding to the egg whites. But if you do that it will lower the temperature of the syrup and I'm not sure it would be warm enough to kill any bacteria.

On the butter temperature, I have found by trial and error that if the butter is at soft room temperature when you beat it in the mixture breaks and becomes soupy or curdled, it can be brought back together by chilling the icing and rebeating. But if you add the butter when it is brick hard you will get little lumps of butter in the icing that won't smooth out.
So, I have found it works best for me if I cut the butter into pieces, leave it in the refrigerator while the sugar syrup is cooking, as you add the syrup to the egg whites and beat till cool, that is when I take the butter out of the fridge. It works best for me if the butter is cold but has a bit of give to it if you squeeze the wrapper. Here is the recipe I use.
http://cakecentral.com/cake_recipe-2426-Italian-Meringue-Buttercream--Shirleys-Method.html

Okay, another question. Do the egg whites need to be at room temp before using?

I have used the whites at room temp. but sometimes forget to take them out in time and they are fine even cold from the fridge.

Thanks Shirley! I guess I am going to try the recipe and cook the sugar to 238 since that seems to be the most commen temp recommended. What attachment do you use, the whisk or the paddle?

re: IMBC...
there are a lot of different recipes out there...

i heat the sugar to 248-250. heating it above 250 is not good because adding sugar beyond 250 will break down the egg whites' structure, leaving a less stable final buttercream. once all the sugar is dissolved they say not to stir it anymore. (you don't want the sugar to re-crystallize)

cold eggs separate easier, so as soon as i take the eggs out of the fridge, i separate them. but using egg whites at room temperature form a meringue faster. (ie, it takes longer for cold egg whites to form stiff, but not dry peaks). i use the whisk attachment to beat the egg whites.

also, there is still a risk for salmonella with this icing. the temperature of the egg whites does not get above 160 (the temp needed to kill salmonella) if this is a concern, i was told to use pasteurized egg whites.
(or make a swiss merigue buttercream, which uses a cooked meringue)

also, i add butter which is at room temperature, and just add one tablespoon at a time, and beat until each tablespoon is incorporated.

If the icing starts to curdle, just increase the speed on the mixer to high, and the icing should re-emulsify.

Hope that was helpful...

Thanks for all the tips. I ended up making it and am very happy with it!! I cooked the sugar to soft ball stage, 238, as this seemed to be the most popular temp suggested. I used butter that I let sit out for a bit that had softened but still had a chill to it. My IMBC turned out perfectly! It never curdled or got weird looking or anything, it all came together beautifully. I am so glad that I decided to try it. I never could have done it without this site and all the advice and tips!

that's great that it gave you no trouble!

I agree that it does go on like satin!
the swiss meringue buttercream is very similar!

Heat to 248-250 once it hits 248 I pour sugar in clean glass measuring cup to stop cooking. Just like Shirley I cut butter into small pieces left in fridge until I'm ready to pour sugar into whipped egg whites.GL

Shirley, I made it to use on cupcakes so I have not yet been able to see how it smoothes. I have heard that it smooths so nicely, I really hope it does for me because I have the worst time getting my icing smooth! I just wanted to thank you in particular because although I found many directions and recipies, yours was the most clear and precise. I never would of thought I could leave out a stick of butter and it would still turn out. I have heard so many people say it tastes like a stick of butter so I was afraid to try it. I did use 1 1/2 cups sugar and 1/2 water for my sugar syrup to make it a little sweeter but think I will try what your recipe calls for next time just to compare. I just find it so strange that the cooking temp of the sugar varies so much, from 238 all the way to 250! I am always going to stick with 238 though because it worked great for me!
Thanks to all of you who helped me out!