Beat the egg yolks with the sugar, flour, and a pinch of salt for 2-3 minutes until the mixture is pale and thick.
Heat the milk in a small saucepan or microwave until steaming (about 60-70°C / 140-158°F).
Add the hot milk and lemon zest or vanilla pod to the egg mixture, then whisk together for 1-2 minutes until well combined.
Transfer to a saucepan and bring to a boil over high heat, stirring constantly for 3-5 minutes until the mixture thickens enough to coat the back of a spoon.
Remove from heat and remove the lemon zest or vanilla pod.
Either sprinkle with sugar and caramelize using a kitchen torch or hot iron until golden, or simply sprinkle with cinnamon.