Preheat oven to 350?F. Grease and flour a 10-inch Bundt pan. Cream butter and sugar; add eggs one at a time. Gradually add powdered sugar (yes, dry powdered sugar ? key to fudgy tunnel). Stir in flour and cocoa until combined. Fold in a generous amount of chopped walnuts. Pour batter into pan and spread evenly. Do not overbake: bake ~45?50 min ? the top will set but a ?tunnel? of gooey fudge will remain in center. Cool in pan 1 hour, then invert. The interior will be soft and fudgy. Serve slices as is, or with a dusting of powdered sugar.