In a large saucepan mix sugar and cocoa well then slowly add evaporated milk. Cook over medium heat until mixture starts frothing up then lower heat til mixture boils evenly. Cook approximately 25 minutes to a soft firm ball stage. (A drop of mixture dropped into a glass of cool tap water will hold the shape of a soft ball and will be firm but not hard.) Stir in butter and vanilla. Remove from heat and add marshmallow. Stir until completely melted. Add pecans and stir well. Beat mixture with a large spoon until it begins to stiffen. Pour into a buttered 3x9 pan. Spread evenly. Work quickly. Let cool and cut when somewhat cooled. NOTE: For best results do not make this fudge on a humid day.