A brown sugar/butter/flour base topped with melted bittersweet chocolate and walnuts. Dorie's recipe suggests topping with caramelized rice krispies, but I wanted to freeze most of these and chose walnuts instead. Based on other baker's comments I lowered the oven temperature to 350 and baked for only 15 minutes, leaving me with a crust that was very easy to cut, and so sweet. These bars are more like candy. So easy to make. I hope to try them again with the caramelized rice krispies soon.