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.
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.