In a medium saucepan, combine chocolate and 1/2 cup of the milk over low heat. Stir often until melted.
Once the chocolate is melted and smooth, slowly whisk in remaining 2 1/2 cups milk. Remove about 3 tablespoons to a small bowl. Whisk the cornstarch into the small amount of liquid until dissolved. Pour into the saucepan with the sugar. Stir constantly until the chocolate thickens.
To make the whipped cream, pour cream into a medium bowl. Add orange zest and beat until stiff peaks form.
Pour chocolate into mugs and top with whipped cream.