Perhaps adding some glucose syrup to the recipe would help by preventing crystalization of the sugar in the chocolate. Greweling recommends between 10 and 40 percent glucose relative to the weight of cream. I usually use 20-25%. So for your recipe, you could use 1 to 1.25 oz of glucose syrup. You can also use corn syrup instead of glucose syrup (essentially the same thing except for maybe a different water content or dextrose equivalent).
Add the glucose to the cream before heating it so it dissolves into the cream, then add to the chocolate as usual.
