Why did my chocolate icing exude its cocoa butter?

I tried making Corriher's "Luscious, creamy chocolate icing" for the first time.

340 g milk chocolate (I love Hershey's)
225 g semisweet chocolate 
28 g light brown sugar 
a pinch of salt 
5 ml vanilla extract 
45 ml light corn syrup 
366 g sour cream 

In a mixing bowl, stir together the brown sugar, salt, vanilla, and corn syrup. 
Stir in the sour cream with 1 or 2 strokes only. Add the melted chocolate. 
Beat on low until very smooth. 

I did have a few substitutions. First, I used pure semisweet chocolate only, no milk chocolate. Second, I used creme fraiche, because it's richer than sour cream (and because I seldom have sour cream). Third, I used agave syrup and white sugar, because I can't buy corn syrup, and figured that with the taste from the agave syrup, I don't need the brown sugar. I can't get the usual American type here anyway.

I also didn't think of taking out the creme fraiche early enough, so it was at maybe as low as 15 Celsius when I made the icing. But note that the recipe doesn't say anything about temperature. I also overlooked the "mix on low" part and mixed on medium speed at first.

The icing started out just as it should. But during mixing, it expelled the cocoa butter much in the same way that just-a-little-overheated mayonnaise expels its oil. Basically, I ended up with a good mass of icing swimming in a puddle of melted cocoa butter. The chocolate was not seized, distempered or anything else - a few drops of it on the counter hardened to perfect consistency. I was luckily able to save it by heating it again to 35 Celsius, adding two pinches of lecithine, and mixing again on high.

Does anybody know what contributed to the problem and what didn't? Can I expect that it will work next time if I only use warm creme fraiche? Or do I also have to use sour cream, is the liquid in the creme fraiche maybe not sufficient? Or could it be that this type of icing only works with emulsifiers, and it works for the author because Hershey's is full of them and my european organic chocolate doesn't have them?