Cream sugar, butter or margarine and vanilla flavoring until light and fluffy. Stir together flour and cocoa; add to creamed mixture and mix well. Stir in pecans.
Pour into a greased and floured 9 x 13 inch pan. Bake at 325 for 30 to 35 minutes. You can also bake these in layers, if desired. Cool. Spread marshmallow cream all over cake or cake layers.
In a mixing bowl, cream 4 tablespoons butter or margarine until light and fluffy. Gradually add 2-1/2 cups of the confectionary sugar and cocoa. Blend well. Beat in milk and vanilla flavoring. Add remaining 2-1/2 cups confectionary sugar. Add enough milk (4 tablespoons), if needed to make it spread easy. Frost between layers, top and sides of cake.