Using a non-stick spray, coat a 7x3 springform pan.
Add Oreo cookie crumbs, melted unsalted butter, brown sugar, and mix until it is all coated with butter and damp. (Full graham cookie pieces can be pulsed in a food processor until they are crumbs)
Pour the mixture into the springform pan and start to smooth out your crust. Start by spreading it somewhat evenly across the bottom, and then using a measuring cup or glass with a flat bottom, pack it down more even.
Place the springform pan in the freezer for 25 minutes.
While your crust is freezing, using a hand or stand mixer on medium, beat together your softened cream cheese and hot chocolate mix.
Then add the remaining ingredients and beat until smooth. If you need to scrape down the sides to ensure everything is well combined.
Remove the springform pan from the freezer and pre-fit a "lid" with tinfoil. Have it loose at the top in case the cheesecake rises to touch it, but tight along the sides. Set this aside.
Pour the batter into the springform pan, ensuring your mixture leaves about ¼ inch at the top, and then place your tin foil "lid" on top.
Add 1 ½ cups of water to the Instant Pot, the trivet, and your springform pan (gently) and secure the lid with the valve closed.
In manual mode, set your timers to 40 minutes; on high if you have the option and press start. When the machine is done, allow the pressure to release naturally for 25 minutes before opening the lid and removing your cheesecake.
Place the cheesecake on a cooling rack, remove the tin foil for about an hour, and then scrape around the edges with a sharp knife. (The center may look a little jiggly, but it will change as it cools. If it does not, you can put it back in the Instant Pot for 10 minutes with another 10-minute natural release before opening the lid).
Move the cheesecake to the fridge and allow it to set overnight or at least 6-8 hours before removing it from the springform pan.
Serve as is or top with marshmallows or cocoa powder.