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 hand blender too. Strain the corn cream through a fine mesh strainer back into the saucepan and set over medium-low heat. Add the sugar and salt and cook, whisking constantly, until the sugar is fully dissolved, about 2 minutes.
In a small bowl, whisk the remaining 1/4 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 lightens in color, about 1 minute. 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.