Place eggs, sugar, lemon juice and cornstarch in a shallow saucepot and whisk to combine. Place saucepot on the stove top and turn the heat to medium. Whisk the mixture until it starts to bubble around the edges, about 6-8 minutes. Whisk in the heavy cream and limoncello. (The mixture should thicken up considerably when you add the cream.) Remove the pot from the heat.
Set a sieve or fine-mesh strainer over a bowl and pour the warm mixture into the sieve, using a rubber spatula to push the mixture through to ensure the mixture is very smooth. Discard any small bits left in the sieve. Divide the silky mixture evenly between 4 8-ounce ramekins. Place the ramekins into the fridge and chill for at least one hour.