Put the sweet potatoes in the steel insert of your instant pot and add water to cover them. Secure the lid and make sure the steam release valve is sealed. Press the pressure cooking button (high), and set the timer for 8 minutes. Once the timer beeps, release the pressure after 10 minutes. Take out the sweet potatoes from the water and remove the skins. Using a fork, mash the soft flesh.
Transfer the sweet potato mash back to the instant pot and pour half a cup milk or water. Using an immersion blender, blend the mash until it is quite smooth. Get rid of all the lumps while pureeing it. If you don't have an immersion blender, you can do use a regular blender too.
Add in the rest of milk, mix well using a large spoon, and set the instant pot to porridge mode (30 minutes). Walk away. When the pot beeps, wait for a natural release of pressure, then remove the lid.
Press the sauté button (high) and cook the milk and sweet potato mixture until it reduces and thickens, stirring frequently (30-40 minutes). Add the sugar, saffron, and powdered cardamom, and cook for 5 minutes more. Cancel sauté.
Take out the kheer in a bowl. Add the kewra water if using, stir, and allow the kheer to cool down. Keep the bowl in the fridge for at least 3 hours and garnish with slivered almonds, pistachios, and rose petals before serving.
Kewra water is an extract of pandanus flowers. A clear, fragrant liquid, it is used for flavoring desserts and biryanis in North Indian cuisine. It is readily available in Indian or Pakistani grocery stores. You can substitute it with rose water if you wish.
For stovetop instructions, see the body of the post - Stovetop Option.