Preheat oven to 350 degrees F. Microwave butter and chocolate on high about 2 minutes until melted. Add each remaining ingredient, stirring after each addition. Gently fold in the walnuts and chunks of chocolate. Pour into a 9 by 13-inch buttered baking dish and bake for 30 to 35 minutes, until toothpick inserted in center comes out with fudgy crumbs. Do not overbake!
Recipe courtesy of Woullet Bakery, Minneapolis