Eggs Benedict using Goats milk butter
It's hard to know exactly why your sauce failed, but I can think of two main reasons-- first, your heat was too high; and second, you added the butter too fast.  The use of goat's milk butter should not be a problem, and the amount was fine.  Next time, here are a few things to try, if you haven't already:


When adding the butter, drizzle it in with the smallest stream you can, while whisking as vigorously as you can.  The rate of addition is critical; going too fast will overwhelm the process of creating an emulsion.  It is easier to do with two people, until you are comfortable with the technique.
Keep the heat down.  You need the water to give off enough heat so that the butter stays melted, but not much more.  For that, the water can be just short of a simmer (60-80C/140-176F).  If your water is boiling, you may never be able to keep an emulsion. 
In fixing a broken sauce, technique matters.  Be sure the extra yolk is well beaten before adding the broken sauce.  Next, drizzle in a little buttter to start the emulsion.  Then, as before, drizzle the broken sauce into the yolk very slowly.
Try adding a little water, say, 1-2 tbsp.  Too high a proportion of fat to water will overwhelm the emulsion.  I don't think your recipe is clearly out of proportion, but the amount of water is low enough to suggest it might be tricky to keep the emulsion stable.