I want to make the Devil's Food cake from the Bravetart Cookbook (my favorite cookbook) and use the Milk Chocolate version (p. 115) of her Marshmallow Buttercream. The instructions say to melt the chocolate and whip it into the finished buttercream. However, it does not say what temperature the chocolate should be. I have had a couple of bad experiences in the past when adding melted chocolate to buttercream when the chocolate turned into hard bits throughout the frosting. I don't want that to happen with this buttercream. I tried to find online what temperature the chocolate needs to be before adding it, but the only info I can find is that is needs to be "room temperature", and that is vague. What temperature should the chocolate be so that it won't turn into hard bits when I mix it into the buttercream? Thanks.