Melt butter. Add cocoa and white sugar; mix on medium speed for 1 minute. Add eggs one at a time, mixing on low speed after each addition. Add flour and vanilla. Add salt and mix 2 minutes on medium speed. Stir in walnuts.
Pour into greased and floured 9x12-inch baking pan. Bake at 325 to 350 degrees, depending on oven heat, for 25 to 30 minutes. Cool and frost with fudge frosting.
Melt butter in metal pan on low heat. Add cocoa to melted butter; cook for 1 minute, until mixed and dark. Stir in milk. Cook 1 minute more.
Remove from heat and add powdered sugar. Mix well until thick. Depending on consistency you may need more powdered sugar or 1 more teaspoon of milk. Pour on brownies and spread quickly before it sets.