Slice the kernels off the corn cobs. You should get about 2 cups (500 ml) corn kernels. Break or cut each corncob in half.
In a medium saucepan, pour 2 cups (500 ml) of the milk and the cream, then add the corn kernels and the corncobs. Warm over medium heat until it just starts to bubble around the edge (no need to bring it to a boil). Turn off the heat, cover, and let sit for 15 minutes.
Fish the corncobs out of the warm corn cream and discard. Blend the corn cream to as smooth a consistency you can reach. Using a high-speed blender provides the best results, but you can use a regular stand blender or a stick blender too. Strain the corn cream through a fine mesh strainer back into the saucepan and set over medium-low heat. (You can save the corn solids and add them to a smoothie or a soup.) Add the sugar to the corn cream and salt and cook, whisking constantly, until the sugar is fully dissolved, about 2 minutes.
In a small bowl, whisk the remaining ¼ cup (60 ml) milk with the cornstarch. Whisk into the corn cream. Cook, whisking constantly, until the mixture thickens slightly, about 5 minutes.
Place the egg yolk in a medium bowl and whisk until smooth. Temper the egg yolk by slowly pouring one ladleful of the hot corn cream into the yolk, whisking constantly, then slowly pour the egg yolk mixture back into the saucepan, whisking to combine.
Remove from the heat. Let cool to room temperature, then transfer to an airtight container and refrigerate for at least 3 hours, or preferably overnight.
Whisk the cool corn custard for a few seconds. Pour through a fine mesh strainer into the bowl of an ice cream maker (straining the mixture one last time will ensure a silky-smooth sweet corn gelato). Freeze according to the manufacturer’s instructions. Stop the machine when the gelato is icy but still soft.
STORAGE: Transfer the sweet corn gelato to an airtight container and freeze until firm, about two hours.Sweet corn 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 it as quickly as possible after churning.
SERVING: Always take sweet corn gelato out to room temperature 20 minutes before serving to make it easier to scoop. This will also allow you to enjoy the treat 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.