In a small saucepan heat the cream to a simmer. Remove from heat and add the chocolate and nutella. Swirl the pan to cover the chocolate, allow to sit for a couple of minutes. Gently stir the mixture until completely combined. Pour over the bars and spread to cover evenly. Refrigerate until the topping is set.
Once the bars are set, use a sharp knife, dipped in hot water to cut them. I start by cutting around the edges. This is known, in the professional culinary industry, as "chef's snack."
Cut the bars using the sharp knife, dipping in the hot water to melt through the ganache, this will prevent it from cracking. Into 24 pieces.