Whisk together the heavy cream, whole milk, sugar, vanilla, and salt until the sugar is fully dissolved.
Pour into a Ninja Creami pint container, leaving ½ inch of headspace. Seal and freeze flat for at least 24 hours, until solid.
Remove the lid, place the pint in the Ninja Creami, and run the Ice Cream function.
If crumbly, add 1–2 tablespoons of milk and run the Re-Spin function until smooth and creamy.
Make a well in the center, add the chocolate cake pieces and 1 tablespoon fudge sauce, and run the Mix-In function.
Scoop into bowls, drizzle with the remaining sundae sauce and dark fudge, and swirl gently. Serve immediately.