Break up the chocolate bars into small chunks and place them into a small microwave-safe bowl. Microwave the chocolate in 30 second intervals, stirring after each interval, until the chocolate is melted and smooth.
Add the cocoa powder into the melted chocolate and mix until well combined. Set aside.
Add the cream cheese, sugar, and cornstarch to a large mixing bowl (or to a stand mixer fitted with the paddle attachment.)
Use a hand mixer (or stand mixer) to beat the ingredients together for 2 minutes, until smooth.
Add the four eggs and vanilla into the cream cheese, and beat the mixture on low speed *just* until the eggs have been fully incorporated.
Add in the heavy cream and melted chocolate mixture, and again beat on low speed just until the mixture is cohesive.
Pour ⅓ of the cheesecake filling over the crust. Place dollops of the marshmallow creme onto the cheesecake, then use a knife to very generously swirl the marshmallow into the cheesecake batter.
Pour the remaining cheesecake filling over the marshmallow creme. If any marshmallow is peaking through on top of the cheesecake, swirl it back into the batter.*
Tap the pan on the counter a few times to help remove any air from the filling.
Fill a 10×15" baking dish, or any larger pan that can reasonably fit the 9" springform inside of it, a third of the way with hot water. If desired, wrap tin foil around the bottom of your springform pan to avoid the possibility of water seeping into the springform.
Place the springform pan into the larger pan, then place the whole thing into the oven. Bake for 70-80 minutes, or until the cheesecake only slightly wobbles and reaches an internal temperature of at least 170°F / 76°C.
Once the cheesecake is fully cooked, turn off the oven and leave the door slightly ajar with the cheesecake in it for 1 hr. before transferring it to the fridge to chill for at least 8 hours.