Step 1
In a medium pot over medium heat, bring 2 cups of milk to a simmer (about 5 minutes), then remove from the heat.
Step 2
In a medium bowl, whisk together the sugar, cornstarch, cocoa, and remaining milk, then whisk the mixture into the pot of hot milk. Turn the heat to low and continue cooking until the sugar has fully dissolved, about 3 minutes. Set aside to cool to room temperature, then cover with plastic wrap and refrigerate for 8–24 hours.
Step 3
Pour the chilled mixture into an ice cream maker, and process according to the manufacturer’s instructions. Scrape the gelato into an airtight container and freeze until set, at least 4 hours. Scoop into bowls and serve immediately.