In the bowl of a stand mixer affixed with the paddle attachment, start mixing the cream cheese on the lowest setting. Use a rubber spatula to scrape it down and press it until you cant find any lumps. Add the brown sugar, salt and vanilla and mix well.
Add the hazelnut butter, scrape down the bowl and mix until well blended.
Add the flour, and eggs and while the mixer is on, pour in the heavy cream. Mix to just combine.
If you see any lumps in the cream cheese at this point, consider running the batter through a fine mesh sieve to remove them. There will be very tiny 'bits' from the hazelnut paste but those are fine to keep in the filling (and they add a hint of texture).
Pour the batter into the cooled crust. Fill a 13 x9” cake pan halfway with water. Place the cheesecake pan in the water and set it in the oven.
(If you see any dark spots starting to form on top it’s probably from some hot spots on the roof of the oven, gently nudge the pan away from those areas.)
Bake for 60-75 minutes, until you can gently shake the cheesecake pan and most of it looks set. The middle will still have a bit of jiggle.
Turn off the oven and let the cheesecake cool while still inside. Prop a wooden spoon in the door to allow it to cool. After about a half hour you can take the cheesecake out and set it on the counter. When it’s at room temperature, transfer to the fridge. Chill overnight, or at least for 8 hours.