Place chopped chocolate in a heatproof bowl; set aside.
Combine heavy cream, milk, and cocoa powder in a medium saucepan over medium heat. Whisk until cocoa is dissolved and the mixture comes to a simmer. Remove from heat and pour over chocolate. Let it stand for 2-3 minutes. Whisk until smooth, stirring in vanilla. Pour through a fine-mesh strainer in case the cocoa powder was clumpy. Divide mixture evenly among the molds
Cover mold, add sticks and freeze for at least 5 hours or overnight.
TIP: Run the molds under warm water to loosen up the popsicles.