Desserts! help!?

I am making a candy bar but don't want to use the usual chocolate. I want to replace the chocolate with dulce de leche. My question is how to make the dulce de leche harden over the bar. I don't want it to be so i have to constantly refrigerate it to keep it hard. How do candy bar company's keep the chocolate hard without refrigeration? Can i do the same with dulce de leche?