I have just started using SMBC in the past week. I made four batches in the last three days. Here has been my experience on this.
1. I use the recipe posted here on page 2, the sin SMBC or something like that listed in the recipe section. The proportions work for me and it can easily be doubled.
2. I use a candy thermometer. I agree with mamawrobin. They are only $4 bucks at your local grocer. Mine has a clip too, but doesn't work well with my bowl. So I just have it resting on a towel next to the stove and stick it in and check it. In the sinful recipe it states 3 minutes, which is about what it takes to get to about 150 degrees. So I set the timer just so I have a reminder of how long I've been working on this part.
3. My CIA book states that between 140-150 is enough to sterilize the egg whites without them starting to cook. So I strive to 150 degrees.
4. I used liquid egg whites twice and got OK results. By OK, I mean that the first time the SMBC was not stable enough.It barely stayed on the cake. At the time, I blamed it on the heat of the day. I didn't use the thermometer, I just cooked it for three minutes. I didn't let the meringue cool and get to stiff peaks before I started adding the butter. Based on those two errors I used the liquid pasteurized eggs again for the second batch. I got a bit more stable SMBC, but never got the eggs to make into a meringue. Then someone on her mentioned on how it states on the side of the box that liquid eggs will not whip into a meringue. So I made my third batch using real egg whites and a thermometer. This batch was much, much better. I added white chocolate to the first two batches and did a taste test before adding the white chocolate. This even tasted less butter like.
5. The fourth batch is the best and what I will do from now. Still using the ratio of the sinful recipe as my guide, I cooked the sugar/real egg whites to 150 degrees. I then used the whip attachment and started whipping the mixture. While it was mixing I took a towel and soaked with the very cold water and wrapped it around the bowl to expedite cooling. In the future I will use ice packs that I just bought. Then when I got stiff peaks in the meringue I, switched the beater to the paddle started adding the butter one tablespoon at a time. It then went through the curdling stage. After the curdling stage was complete the frosting was even smoother and lighter than when it hadn't done this in the first two batches.
The important thing is that at this point with the taste test it tasted smooth, silk like with just the right touch of sweetness. AND you could not taste any discernible butter flavor. So I believe that if you distinctly taste butter, then you have made the SMBC wrong somewhere in the first part. And my guess is the whipping of the meringue is the key. You need to get a nice good stiff meringue just before it gets to the point of being dry.
On my last two batches I also added a Raspberry puree made from fresh raspberries. This is where I think refrigeration is the key. I made a cake on Sunday and used both White chocolate SMBC and a Raspberry/White chocolate SMBC. I left the cake out of the fridge with a saran wrap covering (I have to get a covered cake stand and soon!). Last night, Tuesday night, I noticed the raspberry frosting looked darker than it was on Sunday. I had just had a piece of cake at lunch, so I know it was fine then. But by evening the raspberry had started to turn. The white chocolate SMBC part was fine and the cake was fine, but the raspberry fruit needed refrigeration. So depending on what you use to flavor the frosting it may need to be refrigerated. But cake taste better at room temp, so just take it out for about 20 minutes before serving and then you can put it back in the fridge too.
Sorry for the long message, but I am now a huge convert of SMBC. Done right it is the most heavenly thing you can make.
HTH
. I still end up with waste! Can you use the meringue powder?
But anyway 
I've never tried it but it sounds yummy.
