Don't burn the chocolate or it will cause the top of your Nanaimo bars to be crackly and hard to cut through. If there are some chunks, take it off the heat and let the residual heat do the work. If it's not shiny, it's burnt, don't use it. When drizzling the chocolate on top, do it evenly so you don't have to spread it out that much, and don't mix the custard and the chocolate layer, you want them to stay separate.