Japanese mochi recipes really need to be eaten within the day they are made, as they usually don’t add fats into the mochi dough, which would keep it from drying out (this is different from mochi recipes you may see from Taiwan or other Asian countries). If you use shiratamako instead of mochiko, it will stay pretty soft, even if you put it in the fridge. So if you have a really hot kitchen and your chocolate is going to melt, feel free to stick it in the fridge to be eaten within a few hours. Still, if you leave it overnight, it will definitely harden somewhat and dry out. Hope that helps!