Combine butter and cocoa in saucepan until the butter melts; cool. In a large bowl, add the sugar and eggs to the butter mixture. Mix until blended. Add flour, salt and vanilla and beat until blended. Stir in the pecans if desired. Bake at 350F for 35 minutes in a lightly greased 13x9 pan. Remove from the oven and sprinkle on the marshmallows if desired. Spread the frosting over the marshmallows. Combine all the frosting ingredients in a bowl and beat well until smooth, adding additional milk if frosting is too stiff.