1. Chop chocolate. In a medium saucepan place milk, cream, and 1/2 cup sugar. Heat slowly and stir until sugar dissolves. Turn off heat and add chocolate and cocoa powder, whisking until smooth.
2. In a bowl place remaining sugar, eggs, and cornstarch; beat until thickened. Pour sugar and egg mixture slowly into chocolate mixture, cook over medium-low heat stirring constantly until temperature is 165°F.
3. Prepare a large bowl of ice water. Empty saucepan contents into a heat safe bowl and place bowl in ice water. When cool, cover and refrigerate until ready to place in ice cream freezer.