I'm a new convert to SMBC and my first 5 batches were spot on.  I made a batch last night and it was pretty bad.  The best way to describe it is it tastes like shortening.  It's super airy and looks lot different than the batch that came out good.  In the picture below, the bad stuff is on the left.  I just finished making another batch and it came out exactly like the bad stuff last night and looks just like the buttercream on the left in the picture.  My process is exactly the same, my recipe is the same (just add different flavorings, but tonight I tried to replicate a past success).  The ONLY difference is our temperature has dropped about 10+ degrees so it's considerably colder in the house. I am pretty sure the butter (at room temperature) was colder these two fails than it was on my successful attempts.  Aside from that, I'm truly at a loss and very frustrated.  My recipe calls for 2 C. sugar, 6 egg whites, and 3 C. of butter.  I add the butter about 1-2 tablespoons every 15 seconds.  Any help/suggestions will be appreciated.

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Quick update.....Because I'm anal about this, I had to try another batch tonight.  This time I made sure the butter was room temp before adding.  Whipped it all up and it came out perfect...Then I ran some tests to try and figure out where I went wrong the last two times.  This particular recipe called for adding maple syrup, so I thought maybe I added too much syrup and molasses.  I added a LOT of syrup and molasses to a tiny portion of it and it still had smooth consistency and tasted good (just very sweet).  Next I took a small amount and used a hand mixer and just beat it for a long time, thinking maybe there was too much air in it.  That didn't affect it.  So now I'm back to square one, and the only thing I can think of is the butter was too cold.   I do have some questions:

1) Can you overbeat the  buttercream when you're adding the butter?  It typically takes me about 5 minutes to add all my butter.

2) Can you overbeat OR underbeat once you add your flavoring?  I usually switch to med-high on the mixer and mix for about 3-5 mins once I add by vanilla/extracts/etc.

And not sure if it's relevant, but I am now using a new Kitchenaid 6 Qt mixer and before I had my 4.5 Qt mixer.  Can't really see how that's a factor but it is something that's different.  I'm 1 for 3 on good to bad batches with the new mixer :-(

I find if my butter is not soft enough, it takes too long to mix in and it will look a little curdled, like your pic on the left. You can rememdy that by taking a spoonful of the icing and melting it a few seconds and then drizzle it back into the icing.

When my butter is the perfect temp it will mix in beautifully without going through the soupy curdled stage.  I don't know the temp of the perfect butter but it is soft all the way through without being melted. The temp of my kitchen makes a big difference...the cooler the day the longer it takes for the butter to soften.

Before I start my buttercream I take my butter out and cut it into pieces and place it under my under cabinet lights. When it is time to add it in, I put maybe a quarter of the amount in at a time...if I put it all in at once, it tends to push the meringue up over the edge of the bowl. It does not need to take 5 mins to mix in the butter. 

As far as flavorings, I add mine at the end and just mix a few seconds more, I mix the icing with a spatula, scraping the bowl, once I take it off the mixer.

Thank you, that's some great advice.  We tend to keep our house on the colder side, so room temp here is probably lower than it is for most people.  I like your idea of putting it under the lights, that will be a big help.  Thanks again and have a great day.