When you make hot chocolate in a crockpot, it heats up the milk slowly so that it won’t curdle. The chocolate bars also warm up slowly enough so that they don’t burn or seize. If you have trouble with them melting you can always turn the heat to high for a little while, stir until they melt, and return the setting to low or warm. You can also make a lot of it, without having to worry too much about keeping an eye on it.