Melting chocolate with other ingredients such as milk, heavy cream and butter is not tricky, provided the amount of these other ingredients isn’t so small that the chocolate scorches. When you melt chocolate with other ingredients, you reduce its chances of burning.
There is no formula for how much liquid may be melted with a given amount of chocolate to guarantee that it will not tighten or seize because each brand of chocolate is a recipe itself (and its formula is unknown). From experience, I have discovered that the more fat in the chocolate, the more liquid you need to melt with it. But I’ve had no problem melting as much as 6 ounces of chocolate with 3 ounces of butter or as much as 4 ounces of chocolate with ¼ cup of heavy cream or milk. I prefer to place the ingredient(s) with the chocolate (chopped into small pieces) in a heavy-bottomed saucepan over low direct heat, and stir until three-quarters of the chocolate has melted. Then I remove it from the heat and allow the residual temperatures of the pan and the liquid to complete the melting. When adding water to unmelted chocolate, do not stir right away—allow most of the chocolate to melt first.