Have been making SMBC for quite a while & always turns out great. Last 2 batches have been "watery" once finished. I have used top quality butter that was at room temp & same recipe. Butter was unsalted. Actually had to put it in a strainer with paper towel to get rid of the moisture, as moisture was running down cake as I was icing it. What would cause this? Am at a loss to know why this is happening, when I have had success in the past.
I think it's your meringue weeping/breaking down, I doubt it's the butter. Butter doesn't weep liquid from it's solids............and if it's whipped so it's light and fluffy (like a butter cream should be) it totally can't weep any of it's own water content. The only way butter can appear to weep liquids is if you've added extra liquids to it, like lemon juice, fruit purees, etc... Butter won't incorporate liquids with-out something else in it to absorb the liquids, like sugar, meringue or flour, etc...
I would say you didn't cook your eggs enough, so your meringue wasn't truly stable. Then when you added the butter it broke the meringue down into raw whites. So taste your butter cream........you might be frosting with just straight whipped butter.
Maybe there were some errant egg whites in the bottom of the bowl that did not get incorporated in the meringue? Maybe?
This is a long shot but was the meringue still hot enough to melt the butter and then the water in the butter stayed water and never incorporated. Maybe
All that to say I've sorry that happened it can really be unsettling. But don't worry--you'll get it back.
But I know there is more water in our butter than previously. You mighta got butter froma batch that had a bit too much.
Also when I get Sam's Club buttah and break a stick in half it breaks 'clean'. When I get a grocery store brand buttah and break it in half it's muddier, cloudy, has more milk solids in it and water-- it doesn't break cleanly like the Sam's Club brand and even though they are both grade AA. So even within grade AA there's variance.
And---good save!!! By straining out the liquid.
You didn't get your sugar syrup anywhere near the minimum required temperature. There is no other reason for a meringue buttercream to drip down the side of a cake.
Check your thermometer in a cup of boiling water. It should read 212F or 100C.
If it doesn't, then check the instructions for adjusting the scale. Or else start using the water test as well as your thermometer.
FYI butter is regulated by law to have a specific water content, at the point of production. Even the generic butter sold off label must comply with this rule, although it may vary in the packing process. I seriously doubt it's the butter.
Here's the recipe I use. I just multiplied out the one Margaret Braun gave in her beautiful red Cakewalk book with the gold edged pages.
2 2/3 cup butter
2 tablespoons (not teaspoons) vanilla
1 1/2 cups of sugar
6 egg whites (large eggs)
It's a brilliant formula--I've used it forever. I have it multiplied out to fit all my different mixers. There are a lot of meringue icing booboo threads right now. So for what it's worth--this is an excellent formula.
Check your egg carton. Some liquid egg whites are not suitable for meringue-making - it says on the carton ... this may be one of those brands
I've come across those brands also (although I've never seen where it says so on the carton/I'll be looking for that from now on). I got stuck one weekend using it whites that wouldn't whip fully because I couldn't find any other pasteurized egg white brands. To my great surprise the "off" brand that just wouldn't whip up to full volume wound up working perfectly in my frosting. It doesn't seem logical, but it worked fine! To compensate for the under whipped volume I whipped my butter until very fluffy and then added in the under whipped whites and it held together perfectly after all.
Limpy, I have to go above 140F with those whites with additives. You have to, to get the sugar to dissolve in the whites.
Has anyone ever whipped the butter, whisked the egg whites to meringue/cool stage & then combined them? Just wondering.
Yes I have whipped the butter separately but only to help loosen it up when I was in a hurry. Except I would never whisk (by hand you mean?) anything of that quantity that long. Really the usual way I make it is to let the butter get to soft room temp but not warm, spreadable though --and then to me it's more efficient to add it to the bowl of cool meringue just from the butter wrapper. Otherwise you need to wash the mixer bowl over or have two mixer bowls on hand to do it the way you propose and you're washing double dishes.
So just for efficiency I'd point you to just adding it to the fluffy white (cool not warm at all) meringue. But even if you realize too late that it's warm you can rehabilitate it in the frige for a while and beat hell out of it again. No worries.
But I add my butter like a stick at a time into the meringue while the mixer is going slowly. And an interesting story-- mine never curdled in years and years and one day it did and I forgot that it can happen and I thought the world was coming to an end and I about lost it, wasting all that money on the butter, it was a big batch--fail! Then I remembered about the curdling is part of the process and I was able to talk myself out of the closet my family had locked me in for my own safety of course. 
But seriously you cahn't breath when you see that thing turn into a disgusting creepy mass of sludge and then a few minutes later it's beautiful creamy goodness. Wow
If the buttter is in one pound bricks I cut them into quarters because I don't like my butter to linger around out of the frige. I try to watch the time.
If it is in stick size I lay them out in the wrapper for a few.
Either way I mush down on one to make sure it is not still solid in the middle and then I'm good to go.
SMBC is a process but it's not fragile or tricky it's easily obtained.
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