"After lots of experimenting to develop a low-fat ice cream or gelato, I came up with this recipe. Despite the lengthy instructions, it's very simple to make. I couldn't believe how wonderfully rich and creamy it turned out to be. The secret is in the cocoa powder, which makes the gelato dense and rich without adding fat. Because the gelato contains no fat, leftovers will freeze hard. To soften, defrost leftovers in the microwave for 20 to 30 seconds."
Whisk 1/4 cup milk and cornstarch together in a small bowl until smooth to make a slurry.
Combine the remaining milk, sugar, and cocoa powder in a small saucepan over medium heat. Bring to a boil while stirring constantly. Add espresso powder and the cornstarch slurry. Bring back to a boil; stir gelato mixture until thickened like pudding, about 2 minutes. Remove from heat; transfer to a metal bowl. Stir in vanilla extract and salt.
Fill a large bowl with ice water; set the bowl of gelato mixture inside. Stir frequently until cooled, about 7 minutes. Remove metal bowl from ice bath; cover with plastic wrap. Refrigerate until completely chilled, about 20 minutes.
Transfer gelato mixture into an ice cream maker; freeze according to manufacturer's instructions, about 20 minutes. Pour gelato into a freezer-safe container; cover and freeze until solid, about 10 minutes.