Whisk 1 1/2 cups of the milk into the instant pudding mix, just until combined. Spoon into a plastic ice cube tray, cover, and freeze until solid, about 4 hours.
Seat glass pitcher on the base of the Instant Pot® Ace.
Pour the remaining 1 cup of milk into the pitcher and top with bananas, chocolate syrup, and frozen pudding cubes, in that order.
Secure and lock lid.
Choose the Ice Cream or Frozen Desserts program and pause the program as needed to use the tamper to better disperse the frozen pudding cubes.
Once program has completed, transfer ice cream to a freezer-safe container.
Cover and freeze until your desired consistency, about 1 hour for soft-serve consistency. If fully frozen, transfer to the fridge for 30 minutes to soften before scooping. Serve topped with additional chocolate syrup and chopped walnuts, if desired.
Notes
Helpful Tip: This type of instant ice cream will get creamier as it softens, so do not be afraid to let it thaw before serving. Try topping it with hot fudge or hot caramel sauce.