Heat milk over medium-high heat in a small, heavy saucepan to 180° or until tiny bubbles form around edge (do not boil). Remove from heat; add mint. Let stand 15 minutes; strain milk mixture through a sieve into a bowl, reserving milk. Discard solids. Return milk to pan; stir in sugar, cornstarch, cocoa, and salt. Return pan to medium heat; bring to a boil, stirring constantly with a whisk until mixture thickens.
Place egg yolks in a medium bowl; gradually add half of hot milk mixture, stirring constantly with a whisk. Add egg mixture to pan; bring to a boil, stirring constantly. Cook 1 minute or until thick. Remove from heat; add vanilla and chocolate, stirring until chocolate melts. Pour pudding into a bowl; cover surface of pudding with plastic wrap. Chill. Garnish with mint sprigs, if desired.