Pour 1 cup of milk into a small bowl, add the cornstarch and whisk them well until smooth. Pour the rest of the milk into a non-stick saucepan, adding the sugar and cardamom (and rosewater, if desired) whilst cooking over medium heat. Before it starts to boil, slowly add the milk and cornstarch mixture to the pan, whisking constantly to avoid lumps. Keep stirring until the mixture increases in thickness. Take the pan off the heat and pour into a large shallow serving bowl. Let it cool at room temperature, and then put in the refrigerator for about 1 – 2 hours. Finally, sprinkle your Firni with sliced almonds and pistachios and serve!