Preheat the oven to 320F (160C) and place cupcake liners (about 6) in a muffin tin.
In a small bowl, place dark chocolate and heavy whipping cream. Put saran wrap over the bowl and microwave it in 10 seconds increments and mix. Repeat until the chocolate and the whipping cream are fully combined then set it aside.
In a large bowl, mix softened cream cheese, erythritol, and egg yolk until combined.
Add melted chocolate mixture into the cream cheese mixture and mix well.
Sift cocoa powder into the mixture and mix until everything is completely combined.
Pour the chocolate cream cheese mixture evenly into cupcake liners.
Bake in a water bath for 40 - 50 minutes or until the tops look firm. Once baked, leave the mini cheesecakes inside the oven with the oven door slightly open and let it cool to room temperature (about an hour).
Rest the cheesecakes in a refrigerator for at least 6 hours or overnight.
Dust the cheesecakes with unsweetened cocoa powder before serving (optional) and enjoy!