Preheat oven to 325°F.
To a microwave-safe bowl, add ¼ cup of the heavy cream and the chocolate chips. Heat for 20-second intervals until the chips have melted and the mixture is smooth with no lumps. Set aside.
In a large mixing bowl, beat the cream cheese with a hand mixer until creamy and smooth.
Add the sugar, cocoa powder, and hot cocoa mix, beating until well combined.
Mix in the eggs one at a time, followed by the remaining ¼ cup of heavy cream, mixing until fully incorporated.
Fold in the melted chocolate mixture and stir in the vanilla extract. Pour the batter over the prepared crust.
Bake for 55-60 minutes, or until the edges are set but the center remains slightly jiggly.
Remove from the oven and allow the cheesecake bars to cool completely at room temperature. Then, refrigerate the cheesecake for 2-3 hours, or until fully chilled.
After the bars have chilled, make the hot cocoa whipped cream. Add heavy cream to a mixing bowl. Start mixing at the lowest speed, and then gradually increase the speed to medium until the cream starts to get thicker. (The gradual progression prevents you from spraying yourself with cream.)
When the cream starts to thicken, stop the mixer. Add in the hot cocoa mix. Then, resume mixing again, starting with low speed, increasing to medium speed, and eventually moving to a high speed.
The cream will thicken and start to peak. Mix on high until the cream is thick and stiff. This happens quickly, the stage between soft and stiff peaks, so watch it closely.
Spread the hot cocoa whipped cream over the chilled cheesecake bars. Place the cheesecake bars back in the refrigerator for about 15 minutes before cutting and serving.
Garnish with hot cocoa powder and dehydrated mini marshmallows.