How to Make It
Whisk together cornstarch and 1/4 cup milk in a small bowl to make a slurry.
Combine sugar, half-and-half, and remaining 2 cups milk in a medium saucepan; bring to a simmer over medium. Remove from heat; gradually whisk in slurry. Return pan to medium, and cook, stirring, until mixture comes to a boil and thickens slightly, 8 to 10 minutes. Pour mixture into a heatproof bowl. Cool completely in an ice bath, about 25 minutes.
Gently whisk sour cream into cooled milk mixture. Cover and refrigerate until well chilled, at least 2 hours.
Pour half of chilled sour cream mixture into the freezer bowl of a 1 1/2-quart electric ice cream maker, and proceed according to manufacturer’s instructions.
Transfer gelato to a freezer-safe container; press a sheet of parchment paper directly on surface, and seal with an airtight lid. Place in freezer. Repeat procedure with remaining chilled sour cream mixture. Freeze gelato at least 8 hours or overnight.