if you happen to have evaporated milk in the house, that's actually my preferred substitute for heavy cream. but if not, go ahead and use the half & half, though i'd only use about 3/4 cup in place of one cup of heavy cream. don't try to boil/reduce it because it may curdle or break. just heat it gently (preferably in a double boiler) & melt the chocolate in it.
if you have whole milk & butter on hand, there are two different substitution formulas:
2/3 cup whole milk plus 1/3 cup unsalted butter
3/4 cup milk plus 1/4 cup butter
either one will approximate 1 cup of heavy cream.