In the pot of the Ninja Foodi, add in the sugar, chocolate chips, cocoa powder, and cornstarch. Mix well to combine.
Using a whisk, whisk in the milk.
Set the Ninja Foodi to SAUTE then put on Medium Heat (#3).
Whisk until the mixture thickens and boils, then mix and cook and for an additional 1 minute.
If you are using a thermometer this would be 180*F. I would not recommend cooking it any higher than level 3.
In a small bowl, whisk the egg yolk.
Once the mixture has thickened add a little bit of chocolate mixture/pudding, about 2 Tbsp to the egg yolk while mixing it. This way you are warming up the egg yolk.
Pour the egg yolk mixture into the cooking pot and mix for an additional 1 minute.
Carefully remove the inner pot and whisk in the butter and vanilla. Mix until the butter is fully melted.
Scoop into pudding bowls.
Cool completely and keep in the fridge until ready to serve.
Notes
Pudding can be stored in an airtight container in the fridge for up to 5 days.