In a medium saucepan, combine the whole milk, heavy cream, and lemon zest. Heat over medium heat until the mixture is hot but not boiling. Remove from heat and let it infuse for 10 minutes. Strain to remove the lemon zest.

In a large mixing bowl, whisk together the egg yolks, 1/2 cup of granulated sugar, cornstarch, and vanilla extract until smooth and pale.

Gradually pour the hot milk mixture into the egg yolk mixture, whisking constantly to prevent curdling.
Return the mixture to the saucepan and cook over low heat, stirring constantly with a wooden spoon or heatproof spatula, until the mixture thickens and coats the back of the spoon, about 8-10 minutes. Do not let it boil.

Pour the custard into individual ramekins or a large serving dish. Allow it to cool to room temperature, then cover with plastic wrap and refrigerate for at least 2 hours, or until fully set.

Before serving, sprinkle an even layer of granulated sugar over the top of each custard.
Using a kitchen torch, carefully caramelize the sugar until it is golden brown and crispy. Alternatively, you can place the ramekins under a preheated broiler for a few minutes, watching closely to avoid burning.

Allow the caramelized sugar to harden for a few minutes before serving. Serve the Crema Andorrana chilled.
"Enjoy your meal!"