Chocolate Walnut Bars

"I recently adopted this recipe from the Recipezaar account and just made them last night. They are amazing! Here's what the original poster said about these bars: "These aren't difficult to make, but they look fancy because they have three distinct layers. A shortbread crust is topped by a chocolate layer, which is then topped with a thin walnut layer. The bars freeze well, and they'll keep for several days at room temperature if stored airtight. I've even mailed these successfully, wrapping them in pairs with the bottoms together and packaging them in a sturdy, well-padded carton before placing them in a similarly sturdy, well-padded box. The most important thing to remember about these bars is that the chocolate layer needs some time to set up, even after the bars have cooled to room temperature. I usually make the bars the night before I'm going to cut and serve them, but chilling the bars after they've cooled for about 20 minutes should have the same effect.""