In a medium saucepan, combine the whole milk and heavy cream. Heat over medium heat until it is just about to simmer, but do not let it boil.

In a separate bowl, vigorously whisk together the egg yolks and granulated sugar until the mixture turns pale and creamy.

Sprinkle the cornstarch over the egg mixture and whisk until fully incorporated.

Once the milk mixture is hot, slowly pour a small amount into the egg mixture while continuously whisking.

Return the egg mixture to the saucepan and cook over low heat, stirring constantly until the mixture thickens, about 8 to 10 minutes.

Remove from heat and stir in the vanilla extract, lemon zest, and orange zest.

Pour the custard through a fine sieve into ramekins to remove any lumps. Let them cool at room temperature before covering and chilling in the refrigerator for at least 2 hours.

Once set, sprinkle a thin layer of sugar on top of each custard. Use a culinary torch to caramelize the sugar until it is bubbly and brown.