Put aluminum or plastic foil around an 8x8 pan with high rim. Butter foil and set pan aside. Combine water and stevia in a large saucepan. Put over medium heat and stir until stevia dissolves. Add in evaporated milk and clip candy thermometer to side of pan. Keep stirring until mixture boils and temperature reaches 235°F (113°C). Take the pan off the heat and stop stirring. Let mixture cool to 110°F (43°C), then stir in the grated chocolate until it melts. Beat the mixture with a wooden spoon or spatula until it thickens. When you start to hear popping noises (oxygen) while stirring, add in the nuts and stir until spread out evenly. Pour mixture into prepared pan and refrigerate for several hours until fudge has hardened. Cut into 1 inch squares and serve at room temperature.