Natillas (Spanish Custard)

Pour 1 l whole milk into a saucepan. Add the 1 cinnamon stick, 1 teaspoon vanilla extract, and lemon rind. Warm over medium heat until the milk reaches a gentle boil, then let it bubble gently for 5 minutes. Remove from the heat and leave it to infuse for 10 minutes.
Whisk 6 medium egg yolks and 125 g granulated sugar together in a large bowl until thoroughly combined.
Stir the 1½ tablespoons cornstarch into 20 ml whole milk until completely smooth. Whisk the cornstarch mixture into the egg yolks and sugar. Strain all the infused milk through a fine-mesh sieve directly into the bowl, then whisk until combined.
Pour the mixture into a clean saucepan. Cook over low heat, whisking continuously, for approximately 10 minutes or until the natillas begin to thicken. Remove from the heat.
If using the 6 Maria cookies, place 1 cookie in the bottom of each of 3 serving dishes. Divide the custard evenly among 6 dishes, then place the remaining 3 cookies on top of the other 3 portions. If omitting the cookies, simply divide the custard among the dishes.
Sprinkle the 1 tablespoon ground cinnamon over the natillas. Let them cool at room temperature for 15–20 minutes, then refrigerate for 4 hours until thoroughly chilled. Serve cold.