Mix the egg yolk with the cornstarch in a medium bowl until smooth. Mix in 1/4 cup of the milk.
Pour the rest of the milk into a medium saucepan and add the sugar and vanilla extract.
Bring the milk mixture to a boil, then drizzle 6-8 tablespoons of the hot milk into the egg mixture while constantly stirring.
Run the egg mixture over a sieve, then add it to the rest of the milk.
Return the milk to the stove and cook over low or medium-low heat while stirring until thick.
Pour the cream into a container, cover the surface with plastic wrap, and chill.
Notes
The crema should be stored in the refrigerator for up to three days.
Use room temperature ingredients for the best results.
Use a rubber spatula or a wooden spoon to stir the cream to prevent discoloration.
Make sure to constantly stir the cream as you cook it from the bottom towards the surface to prevent it from lumping. If you notice any lumps, run the cream through a sieve.