Melt butter and cocoa together. Remove from heat and stir in sugar and eggs. Mix well. Add flour, salt, nuts and vanilla. Mix well. Spoon batter into a 13 x 9 x 2" pan and bake at 350 degrees 35 to 45 minutes. Sprinkle marshmallows on top of warm cake. Cover with Chocolate Frosting: 1 (1 lb.) box powdered sugar, 1/2 cup whole milk, 1/3 cup cocoa, 1/4 cup softened butter. Combine sugar, milk, cocoa and butter. Mix until smooth and spread on hot cake.