In a medium saucepan, gently warm the whole milk and heavy cream over medium heat until it just begins to simmer. Do not let it boil.

While the milk mixture is heating, whisk the egg yolks and granulated sugar together in a separate bowl until the mixture is pale yellow and has thickened slightly.

Slowly pour about one cup of the hot milk mixture into the egg yolk mixture, whisking constantly. This is called tempering and it prevents the eggs from scrambling. Pour the tempered egg mixture back into the saucepan with the remaining milk.

Cook the mixture over low heat, stirring constantly with a wooden spoon, until it has thickened enough to coat the back of the spoon (about 170°F or 77°C).

Remove the custard from the heat and stir in the cooled espresso. Strain the mixture through a fine-mesh sieve into a clean bowl. This ensures a perfectly smooth base. Cover and chill in the refrigerator for at least 4 hours, but preferably overnight.

Pour the chilled custard base into your ice cream maker and churn according to the manufacturer's instructions. It should have the consistency of soft-serve ice cream.

Transfer the churned gelato to an airtight, freezer-safe container. Press a piece of parchment paper directly onto the surface to prevent ice crystals. Freeze for at least 2-3 hours to 'ripen' and firm up before serving.