2 teaspoons cornstarch
1 and 1/2 cups whole milk, divided
1 and 1/3 cups heavy cream
1/2 cup sugar
1/2 cup unsweetened cocoa powder
1/4 teaspoon fine salt
2 teaspoons pure vanilla extract
In a medium saucepan, whisk together cornstarch and 2 tablespoons milk until smooth; whisk in remaining milk, cream, sugar, cocoa, and salt. Bring to a boil over medium-high, stirring constantly, and cook 1 minute.
Transfer to a large liquid-measuring cup and stir in vanilla. Pour into ten 3-ounce ice-pop molds and insert ice-pop sticks. Freeze until pops are solid, about 6 hours (or up to 1 week). Just before serving, briefly run molds under hot water to release pops.