Combine milk, sugar, cocoa powder, corn syrup and salt in a medium saucepan over low heat, whisking until sugar has dissolved.
Remove mixture from heat and stir in vanilla.
Cool mixture to room temperature by placing the pan in a bowl of ice water and stirring occasionally. (Be careful that the ice water doesn't overflow into the pan.)
Once cooled, carefully pour the mixture into your popsicle molds. Leave a tiny bit of space at the top to allow for expansion as they freeze. Freeze for 1 hour, insert the popsicle sticks, and then continue freezing until solid, about 24 hours.
When ready to serve, run warm water over the molds to release fudgesicles.