Add the rice, butter, salt, allspice and cinnamon stick to a 6-quart Instant Pot®. Turn the pot to the high saute setting (see Cook's Note). Cook, stirring frequently, until the rice is opaque but not browning, 3 to 4 minutes. Add 1 cup of the milk and continue cooking, stirring frequently, until most of the milk has been absorbed and the mixture is thick and creamy, 2 to 3 minutes. Add the sugar and 4 cups water, then scrape in the vanilla seeds, add the pod and stir to combine. Follow the manufacturer's guide for locking the lid and preparing to cook. Set to pressure cook on low for 15 minutes.
After the pressure-cook cycle is complete, follow the manufacturer's guide for natural release. After 10 minutes, being careful of any remaining steam, unlock and remove the lid.
Stir the rice mixture, scraping the bottom to make sure nothing is stuck. Turn the pot to the high saute setting and add the cream and remaining 4 cups milk. Cook, stirring frequently, until the mixture comes to a boil, 3 to 4 minutes. The mixture will be very soupy, but it will thicken as it cools.
Transfer to a large bowl and cover with plastic wrap, pressing the plastic directly onto the surface of the pudding to keep it from forming a skin. Let sit at room temperature until thickened but still warm, 1 to 2 hours.
Stir the pudding before serving. Top with toasted pecans, rum-soaked raisins, warm raspberry jam or cinnamon.
Settings may vary on your Instant Pot® depending on the model. Please refer to the manufacturer's guide.
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