In a medium saucepan, start slowly warming the milk over medium heat until steaming, but not boiling
In a separate bowl, combine egg yolks, maple syrup, cornstarch, salt, and vanilla extract, whisking until completely smooth.
When the milk is steaming but not boiling, remove the saucepan from the heat and ladle half of the milk into a liquid measuring cup for ease of pouring.
Temper the eggs by SLOWLY pouring half of the steaming milk into the egg mixture while constantly whisking.
Pour the tempered egg mixture back into the saucepan with the remaining milk and continue stirring over medium heat until the mixture thickens and starts to boil (large bubbles will form on the surface).
When the mixture has started to boil, whisk for one additional minute, then immediately remove from the heat and pass through a fine mesh sieve.
Place a piece of plastic wrap directly on the surface of the crème pâtissière to prevent a skin from forming and place in the fridge for two hours or until completely cool.