Place the milk and a strip of lemon zest in a medium saucepan over medium heat until the milk is hot. You’ll see the steam rising. Don’t boil. Set aside.
In a bowl whisk egg yolks, sugar and flour until smooth. This mixture must be smooth.
Remove a strip of lemon zest from the heated milk and discard.
Slowly whisk hot milk into the egg yolk mixture. Stir in the vanilla extract.
Pour the mixture back into the saucepan and place over medium heat.
Whisk constantly until the mixture comes to a boil and thickens.
Simmer for 1 minute to cook the flour.
Pour pastry cream into a bowl and plastic wrap with the wrap pressed onto the surface.
Allow to cool for 10-15 minutes then refrigerate until cold.