2 Tbsp semisweet chocolate chips (or chopped semisweet chocolate)
1/3 cup sugar
1 Tbsp cornstarch
1 1/2 Tbsp cocoa powder
1 1/4 cups milk
1/2 tsp vanilla
1/2 Tbsp butter (I omitted this, and all was still well)
Place the chocolate in a saucepan, and heat over a very low flame until just melted (to avoid scorching, it's easiest to just turn on the burner for a minute, then turn it off and let things sit while you gather other ingredients, by which time they should be softened). Stir in the sugar, cornstarch, cocoa powder, salt and milk, whisking until smooth. Raise the heat to medium, and cook, stirring, until the mixture thickens (~5 minutes). Remove from heat, and add the vanilla and butter. Set aside to cool slightly (if you have plastic molds, you'll want to be sure the mixture doesn't have much heat at all left).
Pour the mixture into popsicle molds. If you have the kind with stick handles attached, simply freeze until solid. Otherwise let freeze half an hour, insert popsicle sticks into the semi-frozen mixture, and freeze completely.