In a medium-sized saucepan, combine the heavy cream and whole milk. Heat over low heat until warmed, but not boiling. Remove from the heat.
In a mixing bowl, whisk the eggs until well beaten. Pour two ladlefuls of the hot milk into the eggs, whisking constantly.
Pour the egg mixture into the pan with the milk, return the mixture to the stove and place it over low heat. Cook, stirring constantly, until the mixture thickens slightly and coats the back of a spoon.
Add the mascarpone cheese and fontina to the saucepan, stirring constantly until smooth and creamy.
Remove from the heat and transfer the fonduta to a serving bowl or fondue pot.
Drizzle the black truffle oil over the top of the fonduta just before serving.
Arrange your dippers around the fonduta, and let everyone dip and enjoy!
Notes
To store any leftovers, transfer the fonduta to an airtight container and refrigerate. It can be stored for up to 2 days.