Preheat the oven to 350 degrees.
To make the shortbread: In a medium bowl, toss together pieces of 11/2 sticks butter, flour, 1/2 cup packed brown sugar and salt, cutting the butter into the sugar, and salt with 2 knives, or your fingertips, until it reaches the consistency of small peas.
Place it into a 9- by 13-inch baking pan and pat down with your hands or a spatula to make a firm base. Bake until golden brown, about 20 minutes.
To make the topping: In a heavy saucepan set over medium low heat, cook together 1 stick butter, 1 cup brown sugar and maple syrup, and simmer for about 1 minute. Stir in the walnuts and cranberries. Pour this over the base and spread evenly.
Bake until bubbling, about 20 to 25 minutes. Remove and cool completely in the pan before cutting into bars.
Nutrition information per serving: