Makes 8-10 fudge pops
Have ready 10 ice-pop molds. If your molds are flexible like the ones we use at the museum, set them on a rimmed baking sheet.
Place the chocolate in a large heatproof bowl, add the vanilla extract, and set aside. In a medium, heavy-bottomed saucepan, combine the cream, milk, sugar, cocoa powder, and salt. Cook over medium-low heat, whisking often to break up the lumps of cocoa powder, until bubbles start to form around the edges and the temperature of the mixture registers 180°F to 190°F on a digital thermometer.
Immediately pour the cream mixture over the chocolate and stir with a whisk or blend with an immersion blender until the chocolate is completely melted and the mixture is a smooth liquid (a thoroughly emulsified mixture will yield the most creamy fudge pop). Strain the mixture through a fine-mesh sieve set over a liquid measuring cup.
Pour the chocolate mixture into the ice-pop molds and freeze until solid, at least 4 hours or up to 2 weeks; follow the manufacturer’s instructions for inserting the sticks. If you don’t have ice-pop molds, pour the chocolate mixture into ice cube trays; freeze until partially frozen, about 30 minutes, and then insert a toothpick or short wooden skewer into each ice pop. Continue freezing until solid.
Unmold the fudge pops, dipping the molds into warm water to loosen, if needed, and serve.