In a bowl, mix cornstarch, sugar, and salt with 1/4 cup of cold milk to form a slurry.

In a small pot, heat the remaining milk over low heat, being careful not to boil.

If making chocolate blancmange, add unsweetened chocolate to the heating milk and stir until melted and smooth.

While continuously whisking the heated milk, slowly pour in the cornstarch mixture.

Continue whisking constantly over low heat for 15 minutes.

Ensure the mixture thickens and the raw taste of cornstarch disappears, but do not boil.

Remove from heat and let cool slightly.

Stir in vanilla extract.

Pour the mixture into decorative dishes or molds.

Cover and chill in the refrigerator until set.