1largeegg yolk, or ½ tablespoon (4 g) extra cornstarch
In a medium saucepan, pour 1 cup (250 ml) of the milk and all of the cream, then add the split vanilla bean, if using. Warm over medium heat until it just starts to bubble around the edge (no need to bring it to a boil).
Meanwhile, in a small bowl, whisk the remaining 1 cup (250 ml) milk, sugar, and cornstarch together. Remove the saucepan with the hot milk from the heat and whisk in the cornstarch mixture. Return the saucepan to medium heat and cook, stirring regularly, until the sugar is dissolved and the mixture thickens slightly, 6 to 8 minutes.
Place the egg yolk in a medium bowl and whisk until pale and thickened, 30 seconds to 1 minute. Temper the egg yolk by slowly pouring one ladleful of the hot milk mixture into the yolk, whisking constantly, then slowly pour the egg yolk mixture back into the saucepan, whisking to combine.
Remove from the heat. Pour into an airtight container. (Leave the vanilla bean into the gelato custard while it cools.) Let cool to room temperature, then refrigerate for a few hours, or preferably overnight. The Sicilian-style gelato base must be very cold before churning: this will produce the smoothest, silkiest texture.
Fish the vanilla bean out of the custard and discard.
Strain the Sicilian-style gelato base to produce the smoothest texture. Pour into the bowl of an ice cream maker and churn according to the manufacturer's instructions. Stop the machine when the gelato is thick and icy but still easily spoonable.
STORAGE: Transfer the gelato to an airtight container and freeze until firm, about two hours.Homemade Sicilian-style gelato is at its creamiest and best if enjoyed within 2 weeks. Past that, some ice crystals will inevitably start forming (especially if the gelato is kept in the freezer section of a regular fridge, as opposed to a chest freezer) and the texture won't be quite as smooth. The gelato will still be perfectly edible for up to 2 months, but my advice is: enjoy is as quickly as possible after churning.
SERVING: Always take this gelato out to room temperature 15 to 20 before serving to make it easier to scoop and enjoy it at its ideal temperature and texture.
MAKE IT DAIRY FREE: Substitute lactose-free milk or oat milk for the regular milk, and lactose-free heavy cream or soy cream for the regular heavy cream.