grainy chocolate ganache

How wonderul to have you here Chef Batali!  I have made many chocolate truffles but am stumped when it comes to working with El Rey chocolate, 61% Mijao Dark.  Although I love the flavor, I often end up with grainy ganache when I use it.  I've tried different recipes, making sure the temp of the cream is good...don't know what's going on.

I use 8oz chocolate discos

4 oz heavy cream

2 oz unsalted butter.

I bring the cream to a boil, add the butter to melt and pour over the chocolate.

Have you worked with El Rey Chocolate...is it just more tempermental?

Thanks!