Put 1 (12 ounce) can evaporated milk and 5 cups sugar in a large saucepan. Bring to a boil and boil vigorously for 7 minutes - no less! Stir often. Be sure to time after it begins to boil. Pour this mixture over 1 (10 ounce) package miniature marshmallows, 3 (6 ounce) packages chocolate chips, 1 cup chopped nuts, 1/4 cup butter, and 2 teaspoons vanilla. Stir well until melted and thickens. Pour into 9 x 13 inch (or larger) pan. Chill. Cut into small pieces. Makes about 4 pounds.