Fudgesicles

Have a silicone spatula near the stove and either a heatproof pitcher or glass measure to pour the mixture into the molds while hot (into heatproof or silicone molds) or a bowl to cool the mixture before spooning it into molds.
Combine the sugar, cocoa, cornstarch, and salt in a heavy medium saucepan. Whisk in just enough of the milk to make a smooth paste. Whisk in the remaining milk.
Bring the mixture to a simmer over medium heat, stirring with the whisk -- constantly scraping the bottom, sides, and corners of the pot -- until the mixture begins to bubble a little at the edges. Continue whisking and cooking for 2 more minutes.
Off heat, whisk in the vanilla and scrape the mixture into the pitcher (for immediate filling) or into the bowl to cool.
Fill the pop molds leaving at least 1/4 inch at the top to allow for expansion. If your mold has a cover with openings for sticks, cover and insert sticks. Otherwise, freeze until the mixture is thick enough to hold a stick upright and then insert sticks. Freeze until hard, about 4 hours if the mixture was cool, 6 or more hours if hot.
Line a tray with wax paper. Fill a container with warm water deep enough to dip the full height of your molds. Dip the mold long enough to release a fudgesicle when you pull on the stick. Remove and set fudgesicles on wax paper. Wrap each in a piece of wax paper and/or put them in a re-sealable plastic freezer bag and return them to the freezer until serving.