Combine graham cracker crumbs with sugar and melted butter and pat down on the bottom of a 7-inch springform pan sprayed with non-stick cooking spray. If desired, sprinkle peanut butter cup pieces on top of the crumb layer. Line sides of the pan with parchment paper.
Pour mixture into springform pan on top of the graham cracker crust.
Place trivet in bottom of pot. Pour two cups of water into the inner pot of the Instant Pot.
Set for a manual high-pressure time of 35 minutes. When the time has finished, allow for a natural release for 12 minutes before letting out the rest of the steam.
Carefully remove the pan from the Instant Pot. Allow it to cool completely before placing in the refrigerator. Chill for 4 hours or more before serving.