In a heatproof bowl, combine the chopped chocolate, cocoa powder, heavy cream and corn syrup. Set the bowl over (not touching) simmering water in a saucepan and cook, stirring occasionally, until the chocolate has melted, about 10 minutes. Remove the bowl from the heat and stir in 1 1.2 cups (12 oz/ 375 g) of the yogurt. Let cool to room temperature.
Meanwhile, in another bowl, stir together the remaining 1 1/2 cups yogurt, the sugar and vanilla.
If using conventional ice pop molds, spoon 2 tbsp of the chocolate mixture into each mold. Top with 2 tbsp of the vanilla mixture. Add another 2 tbsp of the chocolate mixture and then another 2 tbsp of the vanilla mixture. Continue layering until the molds are full. Dip a paring knife into each mold and make a few figure-eight motions to swirl the flavors together. Cover and freeze until solid, at least 4 hours or up to 3 days. If using sticks, insert them into the molds when the pops are partially frozen, after about 1 hour, then continue to freeze until solid, at least 3 more hours.
If using an instant ice pop maker, follow the manufacturer’s instructions to fill the molds, layering the mixtures as above, and freeze the pops.
This popsicle recipe is reprinted with permission from <a href="http://www.weldonowen.com/food-drink/baking-desserts/ice-pops">Ice Pops: Recipes for Fresh and Flavorful Frozen Treats</a> by Shelly Kaldunski and published by Weldon Owen.