Combine milk, sugar, cocoa, corn starch, salt and egg yolk in a medium saucepan over medium-high heat, stirring well with a whisk. Cook for 8 minutes or until thick and bubbly, stirring constantly.
Remove pan from heat. Add 1 tsp. vanilla and bittersweet chocolate, stirring until smooth. Transfer the mixture to a bowl; place bowl in an ice-filled bowl. Cover surface of pudding directly with plastic wrap; cool completely.
Spoon chocolate mixture evenly into ice-pop molds. Top with lid. Freeze 4 hours or until thoroughly frozen.
Yields 32 oz. of chocolate pudding, 16 two oz. or 8 four oz. popsicles.