Spread marshmallows and pecan pieces over bottom of buttered 8" x 12" rectangled baking pan. Place over hot water until marshmallows are softened.
Boil sugar, cocoa and milk together while stirring over low heat for ten minutes. Add butter, continue cooking until mixture reaches soft-ball stage in cold(ice) water. Remove from heat, add vanilla and beat until mixture is beginning to thicken. Pour quickly over nuts and marshmallows in pan.
When cool, mark into squares with a sharp knife. Do not remove from pan until cold.
When cold remove squares from pan and place on wax paper in storage container.