Dissolve the cornstarch in 50ml whole milk, leave to rest.
In a small saucepan combine the heavy cream, the leftover milk, the sugar and a pinch salt, place on medium, bring to an almost boil, add the cornstarch milk and cook constantly stirring until thickened. Take off heat, add the cream cheese and nutella, mix until smooth and combined. Cool to room temperature, then transfer to the fridge overnight.
Freeze in your ice cream maker according to the manufacturer's instructions, adding quatered Ferrero Roche candies at the last couple of minutes of churning. Store in an airtight container in the freezer or spoon right away.