Add ricotta, milk, sugar, vanilla extract, and a pinch of salt to your Ninja Creami container. Stir to combine. (You can also blend it up if you prefer, but it's not needed).
Freeze for at least 18 hours on a level surface in your freezer.
Remove from the freezer and process on the Ice Cream setting. Respin if needed.
Create a well/hole in the center of the ice cream and add the marshmallow fluff (or mini marshmallows). Use the Mix-in setting to combine.
Scoop out and top with Rice Krispies.
Notes
I used my favorite ricotta base ice cream recipe for the base, but if you have another vanilla base you prefer, you can use that instead.
If the ice cream is too soft after adding the marshmallow fluff or marshmallows, simply return to the freezer for 10-20 minutes to harden.
If you have leftovers, smooth out the top and refreeze.