Empty the can of evaporate milk into a medium saucepan over medium-high heat.
Whisk in the cornstarch. Bring to a simmer, whisking often. Continue simmering and whisking until the sauce starts to thicken and is consistency of a thin clam chowder. Remove from heat.
Add the cheese and stir to melt. If you need, you can briefly the pan to the heat to help melt it.