Roast the seviyan: heat the ghee in a heavy-bottomed pan on medium-low heat, add the broken vermicelli and fry, stirring constantly, for 3 to 4 minutes until the strands are evenly golden and smell nutty. Take care — pale seviyan can turn too dark in seconds near the end.
Pour in the milk, stirring as it goes in so the strands do not clump together. Bring to a gentle boil on medium heat, then lower the flame.
Simmer for 7 to 9 minutes, stirring often and scraping the base of the pan, until the vermicelli is completely soft and the milk has thickened a little and turned creamy. The kheer should still pour easily — it thickens a good deal more as it cools.
Add the sugar and stir until it dissolves, then simmer for 2 to 3 minutes more. Stir in the cardamom powder and the saffron milk, if using, and switch off the heat.
For the garnish, heat the teaspoon of ghee in a small pan, fry the cashews and almonds until golden, then add the raisins and fry just until they puff up. Stir half of the nuts and raisins into the kheer and keep the rest for the top.
Serve warm in small bowls with the reserved nuts scattered over, or cover, chill for at least 2 hours, and serve cold as a set pudding loosened with a spoonful of cold milk if it has thickened too much.