I made this recipe: http://cakecentral.com/a/whipped-cream-buttercream-frosting and it usually works out great. Today, however, it is really grainy. I've mixed it for about 15 minutes and still it isn't cooperating. Should I add more boiling water and then thicken it with powdered sugar? Mix it longer? Let it sit? Something else? Any ideas would be appreciated! Thanks!

I just checked out the recipe..seems nice... you probably let the water cool a bit before pouring it on the sugar and was not hot enough to melt the sugar which make the buttercream smooth. You can take maybe 1/3 the recipe and get it hot enough to completely melt making sure the sugar is completely melted in the hot mixture...place the grainy buttercream on the mixer and immediately and quickly pour(and carefully) it onto the grainy buttercream and mix from med/low to high speed quickly, so it  does not cool off.Hopefully it will be hot enough to melt the remaining sugar. Let it rest for awhile and remix it to 'refluff ' ..

Might want to check your powdered sugar.  I've noticed a few brands that I've trusted for years have a gritty texture when rubbed between my fingers (right from the bag).  There's just no fixing the frosting if that's the case.
 

A[INDENT][/INDENT] Did you happen to soften your butter in the microwave? I've heard that this can cause a grainy buttercream.

Did you do anything different with the meringue powder? Did you by any chance switch your brand of meringue powder, the quantity of the powder, or how you dissolve the powder this time around?

Thank you for all the replies. I switched brands of egg powder and powdered sugar. I am making it again today and will try dissolving things better.